Very fair point, and a little hard to tackle. I have no solid answer either as to how to get fully correct statistics that actually help in evaluating just how bad the situation is, and where to start working on it. But, as you point out: One must take into account many factors in detail in order to get anything related to reality.
There are also many factors that have no bearing on a given matter. Bringing up local statistics in this instance can be interpreted as deliberate equivocation, which could explain some of the strong reactions to your comment.
Personally i was thinking along the lines of: Maybe the real problem is whichever property of the area they are headquartered in prevents higher amounts of black people from living there. Which may, among others, turn out to be: Companies there are biased against black people, so they're relocated less often. However without looking at the situation closely, any conclusions drawn are premature.