The law of blood found in an egg:
The Torah prohibits one to eat blood of a creature. The Talmud and Poskim discuss whether this prohibition applies also to blood found in an egg. In other words, is the blood in an egg considered the blood of a creature or not. While the final Halachic ruling follows that all egg blood is at the very least Rabbinically forbidden to be consumed due to Maras Ayin, nevertheless, the egg itself is at times permitted to be eaten after discarding the blood. This matter depends on where the blood was found, and whether the egg was fertilized by a male chicken or not.
A. Fertilized egg [Mufaros]:
The status of the blood: Blood [of a fertilized egg] which is found in the area of the start of the formation of a chick [i.e. the Rokeim Haefroach] is Biblically forbidden to be eaten under the penalty [of lashes]. If one is unsure if this blood is from the formation of the chick, it is not Biblically forbidden to be eaten, although the Sages prohibited it.
Must one throw out the entire egg or only the blood: If the blood is found on the yolk of the egg, then the entire egg is forbidden; both the white [Chelbon] and the yellow [i.e. Chelmon]. If, however, the blood is found on the white of the egg, then some Poskim rule one is to discard the blood and the remainder of the egg is permitted. However, other Poskim rule that in certain cases, the entire egg is forbidden. Practically, the custom of Ashkenazi Jewry is to forbid the entire egg irrelevant of where blood is found, whether it is found on the Chelmon or the Chelbon.
B. Unfertilized egg [Muzaros]:
[Blood that is found in an unfertilized egg is only Rabbinically forbidden to be eaten due to Maras Ayin. Thus] if blood is found in Muzaros eggs [i.e. unfertilized eggs], it is permitted to throw out the blood and then eat the remaining egg.
C. Status of eggs today:
The vast majority of eggs on the market today are unfertilized and hence do not contain Biblically forbidden blood. Based on this, some Poskim rule that if blood is found in a typical store-bought egg today, the egg is permitted and the blood is to be discarded. Other Poskim, however, suspect for a minority of fertilized eggs that make it to the market, and hence forbid the entire egg. Practically, one is to be stringent to discard the entire egg, and so is the widespread custom today, although some are lenient to simply discard the blood.
If one finds a blood spot in a store-bought egg, it is proper to discard the entire egg. Nonetheless, some are lenient to simply discard the blood, and permit eating the remainder of the egg.
What is the law if one found a blood spot in a mixture of many eggs?
Coming up in Part 2
 Tur and Shulchan Aruch Y.D. 66; Kerisus 20b; Chulin 64b
 See Kerisus 21a
 Teshuvos Rashba 1:46; Michaber Y.D. 66:7 and Tur 66:7 even regarding Muzaros; Based on Chulin ibid as explained in Beis Yosef 66:7
Other opinions: Some Poskim rule that certain bloods found in eggs are permitted to be eaten. [Riy, brought in Tur ibid, regarding blood found outside of Kesher]
 The difference between fertilized and unfertilized eggs comes down to whether a rooster has been involved or not. Hens do not need a rooster to lay an egg; they do so on their own every 24 hours simply according to light patterns. However, if a rooster does mate with a hen, the eggs she produces are fertilized and, under the right incubation conditions, can bear chicks. If a rooster is not involved it means that there is zero possibility of the egg ever becoming anything more than just that.
 Mufaros refer to eggs that have received male fertilization and hence can develop into a chick.
 Michaber Y.D. 66:2; Tur 66:2; 2nd answer of Tosafus Chulin 64b and Rosh 63; Rashba Chulin 64b; Toras Habayis Aruch 3:5-9; Shut Rashba 1:66
Other opinions: The Mishneh in Kerisus 20b states one is not Biblically liable for eating blood of an egg. Some understand this to apply to all eggs. [1st answer of Tosafus Chulin 64b and Rosh ibid, brought in Beis Yosef 66:2] Accordingly, some Poskim rule that egg blood is only Rabbinically forbidden. [Rambam Machalos Assuros 3:8; Chinuch Mitzvah 148]
 Michaber 66:7; Braisa in Kerisus 64
 Michaber 66:3; Chulin 64b
 Michaber ibid; Tur 66:3 in name of Geonim; Rif; Rambam Machalos Assuros 3
 Michaber 66:3; Rif; Rambam Machalos Assuros 3; Tur 66 in name of Geonim; Those countries that are lenient like the ruling of the Michaber regarding blood found on the white are to discard a peels worth of the egg together with the blood. [Rama ibid; Aruch; Semag]
 Rama ibid “Some opinions rule that if blood is found on the white knot that connects the yolk to the white and the blood slightly extends outside this knot, the entire egg is forbidden. [Rosh; Rashba; Rashi; Tosafus; Tur 66] Furthermore, there are those who are stringent to forbid the entire egg even if the blood did not spread past the knot.” [Issur Viheter Haruch 42]; Furthermore, some Poskim rule that blood found on the Chelbon is more severe then blood found on the Chelmon, and if the blood is found on the Chelbon, everything is forbidden. [Rav Avraham Halevi, brought in Shach 66:8; Taz 66:2; Bach 66; Lechem Chamudos p. 38; Rokeiach 447; Rashal; Peri Chadash 66]
 Rama ibid
The reason: Due to the above dissenting opinion, the custom in these countries is to forbid the entire egg irrelevant of where blood is found, whether it is found on the Chelmon or the Chelbon. [Rama ibid] The reason for this is because we are no longer expert as to what is considered the Kesher of the egg. [Toras Chatas 62:1, brought in Shach 66:8 and Taz 66:2] Alternatively, the reason is because we suspect for the opinion who holds that blood found on the Chelbon is more severe then blood found on the Chelmon, and hence we forbid the entire egg whether the blood was found on the Chelmon or Chelbon, to suspect for both opinions. [Shach ibid; Taz ibid; Bach 66; Rokeiach 447; Lechem Chamudos p. 38; Rashal]
 Michaber Y.D. 66:7; Tur 66:7; Braisa in Kerisus 64b “Eggs which are Muzaros may be eaten by the strong minded”; Rashba Chulin ibid; Toras Habayis Hakatzar 3:5; Yabia Omer 3:2; Yechaveh Daas 3:57; Igros Moshe 36; Minchas Yitzchak 1:106; 4:56
How does blood grow in an unfertilized egg? A blood spot inside an egg do not indicate a fertilized egg. They can be caused by the rupture of a blood vessel on the yolk surface during formation of the egg in a hen’s reproductive system or by a similar accident in the wall of the oviduct. They can also be a result of a vitamin A deficiency, genetics, or some random occurrence. Mass candling methods reveal most eggs with blood and those eggs are removed. However, even with mass scanners, it’s impossible to catch them all. [Taken from https://www.incredibleegg.org/eggcyclopedia/b/blood-spots/]
Other opinions: Some Rabbanim rule that unfertilized eggs are only permitted if the egg has begun to spoil, otherwise, all eggs with blood are forbidden, whether fertilized or unfertilized. [Rav Mordechai Eliyahu, brought in Sefer Kashrus Kehalachah [Edrey] p. 21]
 Rashba Chulin ibid; Toras Habayis Hakatzar 3:5; Beis Yosef Y.D. 66:7
The reason: As egg blood is only Biblically forbidden if it is the start of the development of a chick. Thus, an unfertilized egg which will never develop a chick will never be Biblically forbidden, as such blood can never form into a creature. Nevertheless, the blood itself is Rabbinically forbidden to be eaten and thus must be discarded [due to Maaras Ayin]. [Rashba ibid]
Other opinions: Seemingly according to the Riy, brought in Tur 66:3 the blood is completely permitted to be eaten.
 Muzaros are eggs that have not received male fertilization and hence can never develop into a chick. [Shach 66:14; Beir Hagoleh 66; Tur 66:7; Rashi Chulin ibid; Rashba;] The actual term Muzaros refers to eggs that have begun to rot and become stringy. [Beis Yosef 66:7] Such eggs are also known as Safna Deara, as they are developed from the heat of the ground. [Tur ibid] Based on this latter definition, some Rabbanim rule that unfertilized eggs are only permitted if the egg has begun to spoil, otherwise, all eggs with blood are forbidden, whether fertilized or unfertilized. [Rav Mordechai Eliyahu, brought in Sefer Kashrus Kehalachah [Edrey] p. 21]
How to determine if an egg is not fertilized: Some Poskim rule one can only consider an egg as unfertilized if the chicken who laid the egg was closed up in a coop [for the entire duration of its development], or if there were no male chickens found for a distance of 60 houses. [Toras Chatas, brought in Chidushei Rav Akiva Eiger 66:7] Other Poskim rule that if there is a river without a bridge that prevents the male chickens from crossing then it is also valid. [Minchas Yaakov ibid] Other Poskim rule the chicken must be closed in its coop for 21 days of its development. [Peir Chadash 66:13, brought in P”M 66 S”D 14, Rav Akiva Eiger ibid]
 See Yabia Omer 3:2; Yechaveh Daas 3:57; Igros Moshe Y.D. 1:36; O.C. 3:61; Minchas Yitzchak 1:106; 4:56; Shevet Halevi 2:22; Hakashrus 12:33-35
 In Minchas Yitzchak 4:56 it states that 99% of market eggs are not fertilized
 Nearly all eggs sold commercially are produced by hens that have not mated [i.e. Muzaros], says Lauren Cobey, media representative for the American Egg Board [AEB]. There are no males kept near the female chickens and the eggs are laid without any fertilization. Nevertheless, there are a minority of eggs that are Mufaros. This at times occurs when the egg farmer also develops eggs for chicks and the excess eggs he sells as eggs.
 Yabia Omer Y.D. 3:2; Yechaveh Daas 3:57; Igros Moshe O.C. 3:61 writes that certainly it is permitted from the letter of the law if not found by the Kesher due to the majority, however, see next footnote for his conclusion
 The reason: As although there remains a minority of eggs which are fertilized and sold on the market, nevertheless, certainly one follows the majority in this regard. [Igros Moshe ibid]
 Tefila Lemoshe brought in Pischeiy Teshuvah 66 is stringent by Safek egg; Minchas Yitzchak 1:106; 4:56; Shevet Halevi 2:22; Conclusion of Igros Moshe Y.D. 1:36 that initially one is to be stringent to discard the entire egg, as eggs are cheap. This certainly applies if the blood is found on the Kesher of the egg, in which case one is to be stringent from the letter of the law as possibly such blood can only be found on Muzaros egg; Rav Mordechai Eliyahu, brought in Sefer Kashrus Kehalachah [Edrey] p. 21; Mishneh Halachos 4:96; Teshuvos Vehanhagos 2:384
 The reason: As since there are a minority of eggs on the market that are fertilized, therefore one must suspect for this minority. The reason for this is because it is much more common for a fertilized egg to contain blood, and hence whenever we find blood in the egg we suspect it was fertilized. [Tefila Lemoshe brought in Pischeiy Teshuvah 66:2 Minchas Yitzchak ibid]
 Hakashrus ibid
 Minchas Yitzchak 4:56; Igros Moshe ??: Hakashrus 12:36