This sounds plausibly like something Trump would say in real life. The connection with reality is unnecessary and in fact basically superfluous.
If you had a charismatic Black Boss of DC that made an alliance with the GOP in order to gain power and influence, this would work. A black Ed Crump-style conservative boss who commanded the loyalty of black voters.
Not likely in DC. But in the South, I expect some black elected officials to become the GOP and make alliances in exchange for influence, and I expect some black voters to go along. Black voters reelected Thad Cochran in 2014 and I believe that when the black leaders that provided the margin for Cochran in the primary knock on Cochran's door, they're greeted warmly by a friend who gets things done in Washington on their behalf.
DC isn't all that black anymore, though; gentrification over the past two decades or so has really done a number on DC. In the 2014 mayoral primary, the black "machine candidate" got only 33%, with a black "anti-machine candidate" getting 43% and two white candidates combining for 18%. That black anti-machine candidate went on to win a general election against two ex-Republican independents 55-(combined)42. The primary electorate, and the city as a whole, are probably still narrowly majority black, but the general electorate most likely isn't.
Here's the thing with the sort of white liberals who move to DC instead of New York or San Francisco:
they vote. Even when it's totally meaningless. And note that the city has continued to trend Democratic (from consistently >80 to consistently >90 Democratic) even as its gotten whiter; that also reveals something about the sorts of people who're moving there.
Ivanka Trump is a popular candidate for Mayor of DC after making a lot of local connections during her father's presidency.
Like I said, two ex-Republican independents combined for 42% of the vote in the 2014 mayoral general election. As long as Ivanka doesn't run as a Republican, this may not be such a far-fetched or unrealistic scenario.
A Democrat who promises to put every resident of DC into a concentration camp and nuke the city, while Trump has a >60% approval rating.
Ridiculous, I know, but virtually any Democrat would win DC, even a joke like Dunham.
"No Republican would win" ≠ "any Democrat would win"
This will be even truer after 10 more years or so of demographic change.
