Blackface was a key component of the minstrel shows of the early 19th Century, in which white performers would play slaves and free blacks. These portrayals would be built around insulting, degrading, and dehumanizing stereotypes of black people: the aggressive "buck" with his lustful eye on white women; the freed slave who aspired to polite society but couldn't pronounce his words correctly; the unkempt, neglected black children who were disposable even to their own parents. Blackface demeaned black people, and desensitized white audiences to the horrors of slavery. It was bad news.
https://www.youtube.com/watch?v=UYCaob7MDA8
What does that have to do with you, now that nobody on Earth was alive during slavery? Why is that still a problem in a country with a black president? Why can't I do it, even if I have black friends and several Kendrick Lamar tracks on my iPhone?
You still shouldn't do it, because it's still insulting. It still evokes that long, horrific period in our country's history, whether you want it to or not. It doesn't matter what your intention is, when the thing you are doing is racist. Look at it this way: you might have learned the C word from taking your woke ass to "The Vagina Monologues," but that doesn't mean it's a word you can say around your grandmother.
Without a doubt, there are some people reading this and thinking: "Everyone needs to stop being so politically correct. I'm not politically correct!" To which I reply: Yes, you are. You know how to alter your behavior to move smoothly through life. You know not to grab a female co-worker's breast and say HONK. You know not to smoke on an airplane. You wouldn't go to a dive bar in Wrigleyville this weekend, find the largest, drunkest guy, and yell "the Cubs SUCK" in his face. When the consequences are getting fired or fined or beaten up, you know how to behave. So why not add "considering the history and perspective of another human being whose daily experience is different from yours in ways you cannot imagine" to the list of reasons you should think before acting?