Not to blow out candles with one’s mouth:[1]

The Sages state that one is not to blow out a candle with his mouth, and one who does so is held liable for self-affliction.[2] [Thus, one is to be careful to not blow out a candle with one’s mouth but rather with one’s hand, and the like.[3] Accordingly, when blowing out candles on a birthday cake, one is not to use his mouth to blow it out. Nonetheless, those who are not careful in the above, seemingly do not have to be protested, and on them we would apply the statement of “Shomer Pesaim Hashem” that Hashem guards the fools.[4]]  

For the sake of a Mitzvah:[5] Those women who blow out the Shabbos candle using their mouth[6], may continue to do so, as one who guards a Mitzvah knows no evil.

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[1] Kol Bo 118 [in Din Maaseh Torah which were said by Rabbeinu Hakadosh-Rebbe Yehuda Hanassi] “There are four things that if one does them his “blood is on his head”…3) One who extinguishes a candle with his mouth” “; Reishis Chochmah Chupas Eliyahu 3:7 “There are three things that if one does them his “blood is on his head”…2) One who extinguishes a candle with his mouth”; Pirkei Derabbeinu Hakadosh p. 27 [Shenbloom]; Ksav Yad Ashkenazit [early 15’00’s]; Otzer Hamidrashim Eizanshtat p. 162; Lechem Hapanim Y.D. 116:5 in name of Kol Bo ibid; Zivcheiy Tzedek 116:74; Ben Ish Chaiy Pinchas 18 in name of Arizal in Shaar Ruach Hakodesh; Kaf Hachaim 116:115; See Yabia Omer 9:95-14; Halichos Olam 7 p. 248; Shemiras haguf Vihanefesh 2:267-8;

[2] Literally “His blood is on his head”

The reason: As this causes the Nichfeh illness. [Kol Bo ibid; Reishis Chochma ibid; Nichfe or Kifyon, is a neurological illness similar to seizures-See here https://www.yeshiva.org.il/wiki/index.php?title=%D7%9B%D7%A4%D7%99%D7%95%D7%9F#cite_note-18] To note that a candle is the soul of a person “Ner Havayah Nishmas Adam” and one’s soul is found in his breath.   

[3] Seemingly, on this law we do not apply the rule of “Shomer Pesaim Hashem” to allow people to even initially disregard its warning, as many people still abide by this until today. [See Yabia Omer ibid; See Shabbos 129b and Tzemach Tzedek E.H. 11 that when the public becomes accustomed to perform a danger Chazal state “Shomer Pesaim Hashem; See Igros Kodesh 2:144 that when the danger is associated with matters of Segula [unnatural] then only when people beware from doing it does the Segula danger apply, while if people are no longer careful then the danger subsides completely; See Rashal Yam Shel Shlomo Kol Habasar 12 that today the Ruach Raah no longer resides and only applies in Talmudic times, just as we rule regarding Zugos.]

[4] As this warning is not recorded in the Talmud, Tur, or Shulchan Aruch, and hence gives room for leniency. See Yabia Omer ibid; Vetzaruch Iyun

[5] Lechem Hapanim Y.D. 116:5; Zivcheiy Tzedek 116:74; Kaf Hachaim 116:115

[6] Seemingly this refers to those women who followed the custom lighting the wick and then extinguishing it before lighting, in order so it light well at the time of the blessing. [See Admur 264:14; Tur 264; Rosh Shabbos 2:18] However, possibly, it also refers to those women who followed the custom of not accepting Shabbos with the lighting. [see Michaber 263:10]