If someone actually did that (they'd have to be insane), they'd get immediately lambasted not only by the Leader of their left-wing party, but by the entirety of the mainstream media and establishment for even suggesting that "black supremacy" or something similar to it is even "a thing" at all.

All true. But they should still do it because it's the right thing to do.

Their Leader would immediately fire them (just like Bernier did for the inverse)

And those leaders would be scumbags for doing it, just like Bernier is for doing it to his candidate. Because ethnonationalism is a problem in those parties and sweeping it under the rug accomplishes nothing.