Ask Dr. Racism: Blackface Cosplay is a Dick Move
Dr. David Brothers, PhD in the Truth, spells it out.
4thletter! -- Ask Dr Racism: Blackface, Cosplay, Intent, Reactions, and Responsibility

Intent: When people bring up intent, they’re talking about what someone meant to do, rather than what they did. And that’s cool, I get it. I didn’t mean to be a dick to Asian peoples when I was a kid. I thought I was just having fun with my cousin. My grandmom knew better and put me in my place.
People intend a lot of things, but the only thing that matters is what they actually do. If what you intended to do is show your respect for someone, and you do it by replicating an incredibly dehumanizing practice, guess what! You’re a jerk. You can be a jerk through ignorance as well as malice. And blackface? Kind of a jerk move.
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Shorter version: nobody taking part in that stupid Harlem Shake fad means any harm, but they’re still disrespecting and obscuring the long history of the actual Harlem Shake.