I call this the "Joe Six Pack vs. Darnell 40 oz." factor. Obama does well in areas where either the black population is small, or where it is close to a majority of the Democratic electorate. Where he fails is in areas (New Jersey, New York, possibly Ohio) where there is a large enough black population to cause "tensions" with whites, particularly blue collar white male democrats.
The Joe Six Packs in the NE and Midwest Rust Belt may have pulled the lever for Clinton, Gore, and Kerry, but they are not going to do so for Barry O, for the wrong reasons in many cases.