A Matter Of Resolve
‘The One Who Searches [For Chametz] Recites A Berachah’
R. Yehudah states that one who searches for chametz must recite a berachah. The Ran (ad. loc.) explains that although all mitzvos require a berachah, R. Yehudah nonetheless makes his statement because we otherwise might think that the berachah should be made before burning chametz, not searching for it. Why, then, do we make it before searching for chametz? Because the searching is the beginning of the burning process.
The Gemara states that the proper text of the berachah is “Baruch ata Hashem…v’tzivanu al bi’ur chametz – blessed are You, Hashem…who has commanded us regarding destroying chametz.”
‘Al Bedikas Chametz’
The Ran asks: Since the berachah is recited prior to the bedikah, the text should be “al bedikas chametz – regarding searching for chametz” rather than “al bi’ur chametz.” He answers that since the purpose of the bedikah is the biur, we speak of “bi’ur chametz” (cf., Rosh ad. loc.).
The Rambam (Hilchos Berachos 11:15) writes that as soon as a person resolves to search for his chametz with the intent of disposing and nullifying it, bitul has been effected for at that point he presumably deems the chametz worthless. Accordingly the text “al bi’ur chametz” is particularly appropriate because the berachah refers to the bitul effected at the time of the berachah, as the Rambam writes.
‘Al Bitul Chametz’
The Beis Yosef (to the Tur, Orach Chayim 432) asks why we don’t say “al bitul chametz” since, biblically, the mitzvah disposing chametz is fulfilled with bitul.
He answers that a berachah is only recited on physical acts, not on thoughts. Since bitul can be effected merely by thinking (it does not require verbalizing), we cannot recite a blessing on bitul alone.