One might have expected the opposite. The 99% black areas tend to be impoverished compared to the mixed areas. Think Anacostia vs. Prince Georges County, MD, for instance.
So maybe a strong social network is more important for maintaining health than a higher income? That is interesting.
It is interesting. It could be that a neighborhood with a higher concentation of a single ethnicity includes more married couples with their children and/or more multigenerational families. This would translate into better supervision of children, more meals cooked and eaten at home, more caring people to notice if a person shows symptoms of illness, resulting in overall better health outcomes.