To Purchase this Sefer  The blessing is only recited at the time of wearing of the Tzitzis and not upon tying Tzitzis onto a four cornered garment which one desires to wear.
 Admur 19/1; Michaber 19/1; Menachos 42b
The reason: As the Mitzvah is not completed with the attachment of the Tzitzis, but rather at the time of wearing it. [Admur ibid; Michaber ibid]
Other opinions: Shmuel of the Gemara is of the opinion that all four cornered garments that one wears are obligated in Tzitzis even when one is not wearing them, and they are rather folded in ones closet. [Menachos 41a] According to his opinion a blessing is to be recited at the time of tying the Tzitzis to the garment. [Menachos 42a-b]
The law by a Mezuzah: See M”A 19/1 regarding why in his opinion, by a Mezuzah the blessing is recited even prior to living in the house. See Kaf Hachaim 19/3; Piskeiy Teshuvos 19/2