It’s not the percentage of blacks Obama gets, but how many show up to vote. Hate to sound cliche, but it’s ALL ABOUT TURNOUT. Dem voting in Ohio in 2008 was down from 2004. We lost because the bottom fell out of GOP participation.
The sub-sample of blacks in Ohio and Virginia isn't going to be large enough (maybe 130-or-so each) for the difference to be statistically significant.