Why is there is no bracha on the mitzvah of zechiras Amalek? Shu”T l’Horos Noson () 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. The same logic explains why there is no bracha on the mitzvah of kri’as shema. You say the words “v’dibarta bam” as part of the mitzvah – you don’t need an external reminder. I was wondering if you could use the same to explain why there is no bracha over the mitzvah of haggadah, but on second thought there is a difference – the haggadah does not explicitly mention the text to read. If you never said the pasuk, “V’higadta l’bincha,” I don’t see why m’doraysa you might not be yotzei the mitzvah.
(In addition to the reasoning advanced by the L'Horos Noson, one could argue that it is not proper to say a bracha on the downfall of one's enemies, an idea he discusses in the teshuvah. One could perhaps also argue that the birchas haTorah of parshas zachor also serves as the bracha on the mitzvah of zechiras Amalek, as we fulfill the mitzvah by reading a parsha. And I am sure this is just the tip of the iceberg.)