I disagree. In this case the darts bloke was trying to impersonate a specific black person - he wasn't taking the piss out of her colour. If a black person wanted to impersonate Theresa May then the only realistic way they could do it would be to white up - that wouldn't be racist. This is no...
The challenge being that the joke wouldn't work if he wasn't blacked up as nobody would know who he was trying to represent.
*** edit - beaten to it by our glorious leader ***