i thought there was a substantial black population south of dc, being that it is in dc, and i know there are plenty of blacks in dc (and baltimore).
but, this map suggests that:
(1) this area has few blacks
(2) clinton won this area.
it also suggests that the black population is higher around norfolk, and that sanders won some of those counties. so, my logic may have worked out in the end, despite misunderstanding the demographics of the state.
it seems that clinton win both white and black voters in virginia.
if i'm right, though, that would suggest that sanders should expect to do well in maryland.
http://www.nytimes.com/elections/results/virginia