That's a good point, but isn't it true that according to most polls blacks are pretty strongly supporting Clinton over Obama (by an even larger margin then Clinton's national lead, i.e. blacks are more strongly for Clinton over Obama than whites are)?
The polls that I've seen that actually provide those internals show black voters evenly split for now.
I doubt it would have changed much in terms of actual %, but I can't see any states that Dukakis won b/c of Bentsen.
The states that I'm thinking of which have a significant black population and which Dukakis lost by 3% or less are Illinois, Maryland, Pennsylvania. I know for a fact the blacks didn't bother to show up Election Day in Maryland. I also wouldn't be surprised if in some way that contributed to his massive defeat in Florida - losing Broward County, which hasn't changed that much since 1988.
In other words, the numbers may not have changed much, but having a black man on the ticket certainly could have moved the numbers in these states around a bit (definitely Maryland, less sure about the other two). Sure, Dukakis would have lost Texas by 60-40, instead of 55-45. Big deal.