It's already been explained to you and you still don't get it, do you? Which bit is hard to comprehend? (I'm being genuine here.)
Blackface was a tool that was used by white people to define black people in a racist manner. It was used to spread racial inferiority: black people can only ever be dumb, savages, hypersexual, etc. These damaging stereotypes helped maintain a racially stratified society. When you consider the historical roots of blackface: it doesn't matter what your intention is, when the thing you are doing is racist. Griezmann probably isn't racist... The act itself, however...
(Correct me if I'm wrong, but I remember reading that blackface characters were smart in the beginning, before they went full racist.)
Now over to
@Mister_Stubbs who went running like Jose Aldo after I lectured him. Convenient that he doesn't address facts and ignored my post, yet was brave enough to post this:
Clutching. At. Straws. Because. You. Don't. Have. A. Real. Argument. Classic.
I'll help you out again.
Don't get what's wrong with blackface? Here's why it's so offensive.
This excerpt from the linked article seems to apply to you, unfortunately:
Fwiw, I'm brown, but you've been sniffing too much if you think blackface is remotely funny or a good idea for a "costume". Also, my issue with blackface is that some people genuinely think that in order to be like me, all they need is shoe polish/make up... and that's that. That is extremely offensive to many people because you don't know shit about their struggle, it's much deeper than that. That's why I'm @ your neck. Once you form a coherent argument, I might tone it down... otherwise, please, stop perpetuating nonsense.