In the times of the Beis HaMikdash when the Machatzis HaShekel was given was a bracha recited?
Kovaitz Bais Aharon V'Yisroel 105 page 95 mentions in the name of the Teshuvas HaRashba 18 that a Bracha was not said on the Mitzva of Machtzis HaShekel. However further in Kovaitz Bais Aharon V'Yisroel 105 page 96 it mentions a Yerushalmi Meshectas Shekalim Perek 3 end of Halacha 2 which indicates that a Bracha was said on the Mitzva of Machtzis HaShekel.
Rabbi Yehuda ben Yakar (a/the primary teacher of Ramban) writes in his Peirush HaTefillot VeHaBerachot (page 224 here; subscribers only):
ונראה כמו כן בזמן שהיה בית המקדש קיים והיו נותנים מחצית השקל היו מברכין: אקב"ו לתת מחצית השקל.
...they would say the blessing: Who sanctified us through His commandments and commanded us to give the half shekel.
The Rashba (Shu"t Siman 18) and the Shu"t that are attributed to the Ramban (Siman 189) explain the reason why we do not make a brocha on mitzvos such as Matnas Kehuna and Machatzis Hashekel: Whatever he is giving is not really his, rather Hashem gives it to the Cohen, as it says in Divrei Hayamim (1 29:14), "For all is from You, and from Your hand we have given it to You". (This is in contrast to mitzvos like Trumas, Maasros, Hafrashas Challa and Pidyon Haben were he can make a brocha on the separation, whereas here he would be making a brocha on the giving).