This is a big Machloket Rishonim. The Rambam (Tzitzit 3:9), among others, rules that women may not say blessings on Mitzvot they are not obligated in, while Rabbeinu Tam and the Rashba (RH 33a), among others, rule they may. In OC 589:6, the Shulchan Arukh rules like the former group while the Rama notes the custom is like the latter group (though in his Darkei Moshe there he isn't so excited by this and seems to think that Safek Berakhot leHakel would be better (cf. Bach OC 17)). (In OC 296:8 the Taz argues that the Rama only allowed this for blessings on biblical Mitzvot, but this is not generally accepted.) Modern practices vary.