Depends on who it is. She needs to pick a southern evangelical type, one that can identify with the black, southern, church-going voter. This should secure enough of the black vote to counter Obummer's race cards in borderline states.
As for getting more than 10% or 12% of the black demographic, it ain't going to happen even if she picks a conservative black. Just look at the plantation mentality view of well-qualified conservative blacks like Thomas Sowell, Clarence Thomas, Walter Williams and Herman Cain, just to throw out four examples.