Your guess would be sensible but wrong. While the "Black Belt" is indeed a thing, it is relatively sparsely populated (regardless of race); meanwhile every MSA

Your guess would be sensible but wrong. While the "Black Belt" is indeed a thing, it is relatively sparsely populated (regardless of race); meanwhile every MSA east of the Mississippi, as well as a few in the much whiter part of the country west of it, has a sizable black population, and those are also most of the most expensive places to live in the US. I didn't read as far as methods, but it would probably be no harder to design a study not biased by region than one that is. Maybe easier, depending on how you recruit.