Detroit, maybe, but there's a been a ton of gentrification and building activity in D.C. so I wouldn't be surprised if the population leveled off.
I think the Census estimates are based in part of income tax returns which makes you wonder how many people don't show up in that system, either as having lived there or having left.
Actually, the population in D.C. is increasingly becoming white and affluent while the poor majority black population is now moving to Prince George's County, Maryland.