Just to make one point explicitly clear: even were it to suddenly become explicitly and abundantly clear that blacking-up existed before minstrelsy it still wouldn't make me feel any differently about the practice.

The best way to explain this to people who don't share my uneasiness about it would be to point to the swastika. We know unequivocally, and beyond any shadow of a doubt, that the swastika is a symbol that was associated with Hinduism, Buddhism and Jainism that dates back at least as long ago as 2BC. However, its symbolic associations have been completely steamrollered by its adoption by Nazis. Were anyone to go out with swastikas emblazoned on a T-shirt or a flag or painted on one's forehead, they should not be surprised to be queried, confronted or treated like pariahs

White people blacking-up is overwhelmingly associated with blackface minstrelsy, just the way the swastika is overwhelmingly associated with Nazism. A white person blacking-up is making a statement: "I am aware of all the associations surrounding what I am doing but ultimately I don't care". That "I don't care" is an ideological stance, one I think is crass, embarrassing and actually immoral.