I wouldn't paint my face black regardless on whether I'd heard the term blackface, for a fancy dress party or in any given situation for that matter. It's plain common sense that painting your face black to impersonate a black person or, say, putting make-up on so that your eyes look slanted to impersonate an Asian person is just a no-no.
That being said, I'll have to say I don't like the direction you're going here for it just screams SJW outrage in my book. Griezmann has admitted that he's wrong, brought the post down and apologised for what he did - rightly so - and that should be the end of it. Unlike the Lukaku chant controversy where sadly some of our fans thought it's clever to single Lukaku out intentionally by reinforcing an age-old stereotype of blacks, there's no clear intent (though a racist act is a racist act, intent or no intent) on Griezmann's part to offend black people by painting himself black.
Footballers aren't known for their intelligence and there's just no need to condemn what he did even further. Griezmann should have known better, absolutely, but to give him even more slack for what he did is unfair to him due to the grave implications these days of being deemed a racist. Not many would care to make the distinction that whilst what he did was a racist act, it doesn't necessarily make him a racist.