Actually, the black percentage of the population increased in 1968 and for several decades afterwards because of riots after MLK was killed.
You may be interested in this longish post on my blog on how DC and its close-in suburbs should be admitted as the 51st State, which would finally enfranchise DC residents while simultaneously making Virginia and Maryland more Republican: http://auh2orepublican.blogspot.com/2005/08/fair-and-reasonable-alternative-to-dc.html