Everything that requires a blessing after it requires a blessing before it, but there can be [something] that requires a blessing before it but does not require a blessing after it.

A girl who produced two hairs can perform either chalitzah or yibum, and she is obligated in all of the commandments stated in the Torah.  Similarly, a boy who produced two hairs is obligated in all of the commandments stated in the Torah. He is eligible to be a ben sorer umoreh from the time he produces two hairs until the beard grows around, [which refers to] the lower beard, not the upper one, for the Sages spoke euphemistically.  A girl who produced two hairs cannot perform mi’un. R’ Yehudah says: Until the black appears abundant.

The two hairs that are stated regarding the [red] cow, regarding [tzaraas] afflictions, and those stated elsewhere, [must be long] enough to bend their top to their root; these are the words of R’ Yishmael. R’ Elazar says: [Long] enough to be caught by a fingernail. R’ Akiva says: [Long] enough to be removed with scissors.

[A woman] who finds a stain is impaired, and she must be concerned for zivah; these are the words of R’ Meir. The Sages say: Stains are not subject to [concern for] zivah.