Shelby TN 6%
Dallas TX 4.5%
Montgomery AL 4.5%
Harris TX 3.5% (wowzers!)
Jackson MS 3%
You have to be making some bad assumptions to get these numbers. There are precincts in most of these counties that are all but 100% white, and they don't vote 95% Republican. 80% Republican, sure, but not 95%. In Harris and Shelby (and maybe in Dallas?), there are even a few heavily white precincts that voted for Obama. 20-25% seems like the lowest possible white-for-Obama percent in urban counties (then down to more like 10% in some rural counties in the South).