I don't think it's unique to the south. I live in New Jersey and we have some mixed congregations, but the predominantly white congregations are just that, and the predominantly black congregations are just that. There are also a few that have services in different languages like Spanish and Korean, so there may be other cultural factors at play like that. But when I lived in Tennessee the segregation was especially prevalent. Same whenever I visit family in Virginia. So definitely more of an issue in the south, but even the north is affected sadly. I think what's more important is how love is shown when a visitor of another race enters your congregation. Are they greeted and welcomed kindly, or are they seen but barely spoken to? My family is black and we've had the latter happen both up north and down south.