The DFW Metroplex is up there.
Cross the Dallas County line and you go from Lancaster (among the most heavily AA cities in the region, 69% black), to very heavily white Dallas exurbia in Ellis County. Southern Dallas County is a bit like DeKalb County, GA, except DeKalb's borders are no longer the end to Dem-leaning areas they used to be.
Speaking of which, why is the Ellis-Dallas county line such a big divide?
I think the likeliest reason is that black Americans migrating to the DFW metroplex tend to head north instead of south, while whites leaving suburban areas in the region for exurban ones in many cases might go south instead of north. (Metro Atlanta is opposite in all these respects)
