McCain did that. He got less than 5% of the black vote in ‘08.
Romney did it as well. He got even less of the black vote than McCain.
So are you saying that Blacks (or at least enough to make a difference) are unreachable, and therefore not worth the effort?
What I recall are McCain and Romney going before unrepentant Liberal organizations like the NAACP, more to show that they had (and hence wouldn’t get beat up by the press for doing so). They weren’t evangelizing Conservatism.