Why is no special bracha given to the baal korei before the reading of Zachor (zecher amalek), just like there is for shofar on R. Hashanah?
Several reasons are given for our not reciting a special blessing over the reaing of mechias amalek.
1. Because the reading is part of the broader mitzvah of obliterating Amalek, and as a particular component of a mitvzah, no blessing is recited (see Mikraei Kodesh, Pruim 5). Some add that the final destruction of Amalek will only take place after the coming of the Messiah, and therefore no blessing can be made now (Mishnah Halachos 7:81).
2. Because the mitzvah involves destruction, no blessing is made, because one does not recite blessings over destructive practices (Kaf Hachaim 685:29, quoting from Yafeh Le’lev).
3. Because the mitzvah recalls the initial sin that brought about the coming of Amalek (the nation of Israel were weak in Torah), and one does not recite a blessing over something that causes the recollection of sin.
5. Because the reading of the Megillah also fulfills the mitzvah, and the blessing over the reading of the Megillah includes this.
6. Because the blessing before the reading of the Torah includes this.
7. Because the mitzvah is a permanent mitzvah, meaning that there is a constant obligation to remember the deeds of Amalek, and therefore no blessing is possible.
8. Because the principle mitzvah is in one’s heart (remembering), and therefore no blessing can be recited (see Chinah, mitzvah 330).
See Likkutei Shoshanim, who cites sources for the above.