If you start to discriminate between those instances you choose to find offensive and those you don't then you're already stating that blacking up only 'might' be a racist statement.

It's not a matter of discrimination. White people who have vocational black-faces (chimney sweeps. coalmen, miners, SAS on night patrol etc.) are not engaged in niggering or any other bogus made-up or otherwise re-invented so-called folkloric practise that needs to be explained to the plebs who are wholly ignorant of its noble & wholly non-racist Tradition & History despite the practise post-dating The Black & White Minstrel Show by a good few years and taking place in a racially sensitive multi-cultural society.

I speak as one who used the black-up for genuine folkloric reasons in my South East Northumbrian childhood to go guising at Hallowe'en. And vocational reasons too because we could cause mayhem and not be seen let alone recognised. But we were just kids on the rampage, not grumpy old morris-men cocking a snoop at Political Correctness Gone Mad.