The nusach of birchot hamitzvah
Rashi and Tosfot at the top of 51A disagree about the reason one who is tovel recites the bracha after the t'villa. Rashi explains that the individual was a baal keri who could not make the blessing before the tvilla while still tamei while Tosfot explains that the individual was a ger who was not yet able to make the bracha since he was not yet Jewish.
According to the gemara, the nusach of the bracha is "asher kidshanu b'mitzvotav v'tzivanu [AKB"V] al ha'tvilla." This nusach would appear to make sense. After all, as a general matter, brachot are made over la'asiyatan, that is, before the mitzvah's act is performed. And, in such circumstances, the birchat ha'mitzvah takes the active voice; for example, on Chanukah, the blessing is made AKB"V l'hadlik ner shel chanukah, then the candles are lit. However, when the act is performed first, the bracha is in the passive voice. The classic example is when one washes for ha'motzi, one washes first, then recites "al ntilat yadayim."
It would thus appear that according to both Rashi and Tosfot, since the individual who is about to immerse is unable to make the bracha until after the immersion, the blessing must follow the act and it is thus recited in the passive voice. If that is the case, why is the nusach bracha for candlelighting on Shabbat not similarly passive; after all, the candles are actually lit before the bracha is made since to light them after making the bracha would result in chilul shabbat?