I would think it could be close to offsetting. If it is BO, I wouldn't be surprised if the majority of voting age blacks voted for him (either because they are democrats or because he is black, or both). I believe a general estimate is 15% of the population is black (I'm not sure what % is voting age, though). If we go with the 8% we probably have as many people voting for BO as we do not voting for BO for the same reasons.

Now, is it just as wrong to vote for someone purely based on skin color as it is to not vote for them based on the same characteristic?