Birchas HaMitzvos for Women by Mitzvas Aseh SheHazman Gerama

Rambam Tzitzis 3:10

וְנָשִׁים וַעֲבָדִים שֶׁרָצוּ לְהִתְעַטַּף בַּצִּיצִית, מִתְעַטְּפִים בְּלֹא בְּרָכָה; וְכֵן שְׁאָר מִצְווֹת עֲשֵׂה שֶׁהַנָּשִׁים פְּטוּרוֹת מֵהֶן--אִם רָצוּ לַעֲשׂוֹת
אוֹתָהּ בְּלֹא בְּרָכָה, אֵין מְמַחִין בְּיָדָן. טֻמְטוֹם וְאַנְדְּרֻגִּינוֹס, חַיָּבִין בְּכֻלָּן מִסָּפֵק; לְפִיכָּךְ אֵין מְבָרְכִין, אֵלָא עוֹשִׂין בְּלֹא בְּרָכָה

The Rambam here paskens that if a woman chooses to do a mitzvas aseh shehazman gerama they don't make a beracha. The Raavad argues that they do based on a gemara in Kiddushin 31a. The Gemara there essentially implies that if we hold a blind person is patur from mitzvos he would still make a beracha if he does those mitzvos. So too by women, they would make birchas hamitzvah on mitzvas aseh shehazman gerama. Tosafos there brings in the name of Rabbeinu Tam the same psak as the Raavad.

What will the Rambam do about the proof from Kiddushin? The Gra in Shulchan Aruch explains the Shitas HaRambam with a simple answer. The Rambam holds that even if a blind person is patur from mitzvos on a d'orayssa level, he would still forsure be chayav midrabbanan. Therefore the chiyuv midrabbanan is what is mechayev him in the birchas hamitzvos. Women, on the other hand, are totally patur from mitzvas aseh shehazman gerama. Therefore, by women there is no beracha, even if they opt to do the mitzvah.

The Gra also explains why the Rambam holds that women should not make a beracha on mitzvas aseh shehazman gerama. The reasoning is that in a birchas hamitzvos we say "v'tzivanu" - and He commanded us. Since women were never commanded in these mitzvos, therefore they cannot say this beracha. Why then do we make a beracha on mitzvos midrabban, if Hashem never commanded us? The gemara in Shabbos 23a asks this question and explains that Hashem did command us to follow the words of the Chachamim.

The final line of the above quoted Rambam paskens that in situations of doubt, one doesn't make birchas hamitzvos. This psak of the Rambam is repeated in Milah 3:6 where the Rambam says as follows:

וְכֵן אַנְדְּרֻגִּינוֹס--אֵין מְבָרְכִין עַל מִילָתוֹ, מִפְּנֵי שְׁאֵינוּ וַדַּאי

The Raavad here argues that even in a safek situation the beracha is still made. This machlokes traces back to a machlokes Abaye and Rava in the above mentioned sugya in Shabbos 23a. Some Acharonim say that this machlokes centers around what exactly the issur of beracha l'vatala would be in this case (assuming you weren't supposed to make the berachah). Is it an issur d'orayssa of Lo Sisa or not?