Generally speaking the VP is intended to appeal to a different demographic than the president, so I don't see this happening.
This is true only sometimes. Al Gore and Dick Cheney were not picked because their demographic appeal differed from the other side of the ticket.
Even picks like Joe Biden or Paul Ryan, which offered obvious contrasts with the bottom of the ticket, had little to do with demographic or regional balance. On top of that, gimmicky attempts at demographic balance are responsible for two of the most damaging VP picks in modern electoral history, Sarah Palin and Geraldine Ferraro.
The modern Democratic Party is concerned with this kind of representation to an unprecedented degree, and there would be strong objections to an all-white ticket, but I doubt that a ticket with two black candidates would be a problem, especially if one or both of them had a multi-racial background. There's enough diversity among black Democrats that it could be a balanced ticket in many other respects.