I suspect he'll run with Carson as his VP pick, so I think such duo could win something around 20%, maybe even 30% of the black vote in the general.
Suggesting black people would vote for a candidate just because that person also happens to be black is (one) reason why Republicans get only 5-10% of the black vote.
Republicans get 5% when the democratic candidate for President is black and they get 10 to 13% when the democratic candidate is white.
Obviously, when the republican candidate is black, they should get at least some of tho black democratic vote and i don't expect them to get many - let's say around 20% total. That's not more than 10% of the normal black democratic vote switching sides and voting for a black candidate. That seems very reasonable to me. Plus Trump is well liked by blacks anyway.