When we read/hear parshat zachor on shabbat zachor, we are fulfilling the mitzvah of Remember what Amalek did to you (Devarim 25:17). Why do we not say a bracha on the fulfillment of this mitzvah?
Found the following explanation online. I'm not saying that this is the only reason, but it definitely sounds good.
Shu”T l’Horos Noson (5:47) suggests that the whole purpose of a bracha is to elicit the kavanah that the act being done is l’shem mitzvah. For example, there is nothing inherent in a 4 cornered garment with fringes that suggests it is being worn for a religious purpose – maybe you like the style or that’s the only shirt you have. Therefore, the mitzvah of tzitzis requires a bracha to remind you of its purpose. The mitzvah of zechiras Amalek, however, is different. The very nature of the mitzvah is to remember that there is a command of mechiyas Amalek. You don’t need a bracha to remind you that the act you are about to do is a mitzvah – the very words that you are reading make that point clear.
This question is addressed by many acharonim. Below are a bunch of their answers.
He mentions three possible reasons:
- We don't make blessings on something verbal that entails no action.
- We don't make blessings on destruction.
- We don't make blessings on things which mention our sins, and the episode of Amalek only occurred because we were lackadaisical in Torah.
As per R. Solomon Ben Aderet, we do not make blessings on ruin.
He mentions the reason of not making a blessing on destruction, and rejects it. Instead, he says that there is no blessing because it is already covered by the regular blessing for the Torah reading.
He first quotes R. Haim Palachi that we don't make blessings on destruction, but he doesn't really like this reason so he offers three others:
- It is an ongoing mitzvah with no time parameters so we cannot make a blessing on it.
- R. Solomon Ben Simon Duran explains that there is no blessing for the bedtime Shema because the bedtime Shema is only recited to protect us from danger. Since this mitzvah is also to protect us from danger (from Amalek) we would also not make a blessing on it.
- According to some, the mitzvah can only be fulfilled in the Messianic Era, and even according to those who hold that it can be fulfilled now, remembering Amalek without eradicating Amalek is only a part of the mitzvah, and we do not make a blessing on something which is only a partial fulfillment of a mitzvah.
There are a bunch more, but these are some of the more common answers given.