So, that the function reifies the block is not part of its interface, though, it's part of its implementation. If anything it's a problem that this implementation detail leaks out through the interface.

Furthermore, if you pass it a block with & on the call-side, it's not really reifying a block anyways (even if there is still a performance penalty, which I've never checked), so it's not even true that's what it means in general.

The truth is all functions take a block in ruby. Only some of them do anything with it. Given that, indicating that you take a block with the &block argument seems entirely reasonable to me, if you don't mind the performance hit.