It's a strange one, as the articles on this seem to suggest it was already known about and that he decided to step down due to the fact that inaccurate allegations about it were still swirling around.

The irony of it all is that I doubt any of us would've known about it if he hadn't resigned. The context should factor in too. This happened seven years ago. Ill advised, but the theme of the night was as you say 'favourite musical icon' and also he's done a lot for artists from all communities. Whether jumped it pushed, it's a shame he stepped down in my view and there should be a more sensible approach to evaluating peoples contributions, rather than just 'cancel culture'.

Looked through articles on this across a broad spectrum of publications, and, i havn't seen one mention of him having "blacked up".

He should have claimed he’d gone as Justin Trudeau.

Same, but I don’t really see what else it could be? Given the profile of the bloke and who he was trying to look like.. genuinely open to suggestions.

Seems strange that it's not been mentioned though by any of the sources.
If he did black up then fair enough.
However if he's been forced out just for dressing as a black rapper......

Yep this speaks volumes.