Racist

A reverse would certainly be considered racist. For me, it might depend on more details. Like, if somebody just said, "We're making MLK white", that's strange. My understanding is that Lin Manuel-Miranda wanted to make a point about the immigrant nature of the USA. United Statesians don't tend to think of themselves as immigrants, but they are. With racial tension around immigrants and "Americans", things are getting a little "Gangs of New York". So I think Manuel-Miranda was making a point about that. It was similar to what Baz Luhrmann did with rap/hip-hop music in The Great Gatsby; he was making the music give modern audiences the aesthetic experience that an audience in the '20s or '30s would have had about jazz.

I can't think of a reason why somebody would artistically want MLK to be white. For the record, I think everything's on the table with art, as long as the artist has a reason. People might not like it, but there you go.

That the reaction to "colourwashing" white people is nonexistent or lauded while the reverse - whitewashing - is met with boos: there's kind of a reason for that. Nonwhite performers being erased and ostracised is a big problem in the entertainment industry in the US. Blackface, of course, is also super-racist, and still very touchy. I understand this. I'm reminded of a debate on free speech where Douglas Murray was defending Charlie Hebdo (that he had to! what a world!) and his opponent said that France had laws making Holocaust denial a crime and Murray said in case they weren't aware, France and other European countries have a pretty good reason for being sensitive about that. Maybe it's not an ideal state of fairness, but I understand it.

I'd also say that, unless there was a STELLAR explanation, I would consider blackface to be racist propaganda, and propaganda isn't really art. It's a commercial. In this case, for a repugnant worldview. But I have no problem with certain contexts (The Marx Bros. in A Day at the Races, e.g.)