There is a principle of not saying a bracha in the case where we have reasonable doubt concerning its applicability.
Why then do all kohanim say a bracha when we do birkat kohanim? I come from a long line of extremely secular ancestors, and many kohanim will have something similar in their history. My father and grandfather weren’t even aware of their kehuna and thus would not have been aware of issues such as zonah, and wouldn’t have regarded it as a problem had they been aware of it. Surely the safer option would be to duchan without a bracha these days?
Answer:
You are right that the principle states that in cases of doubt, one does not recite a berachah, and the recitation of a berachah in the csae of birkas ha-kohanim has been cited as a proof the the kohanim of today are considered vadai kohanim — they have a chazakah (an assumption) of being kohanim, and we do not doubt their ancestry without a concrete reason to do so (even secular ancestry would not be a cause for doubt, unless there is a special ‘story’ involved). However, Shevus Yaakov (1:93) and others write that the kohanim of today are only safek kohanim, which begs the question of the blessing recited over birkas kohanim, and also the question of pidyon haben. Some answer this question (see Tel Talpios, Chulin, p. 182) who answers that because there is a safek Torah mitzvah, it follows that there is an obligation to fulfill the mitzvah, at least out of doubt, and this is sufficient to make a berachah on.
