Halacha (Mishna B'rura 57) indicates that a shatz (leader) should not say "amen" to the congragation's "baruch Hashem ham'vorach l'olam vaed" before repeating that line, as the "amen" would be redundant to his repeating the line. AFAICT common practice follows suit.
Yet I think common practice is that the congregation does say "amen" to the shatz's b'racha before Halel, before repeating that b'racha. Why? Is this b'racha different somehow from "baruch Hashem ham'vorach l'olam vaed"? Or is this simply an error?