The internet is still churning over two cosplayers at Emerald City Comicon who wore blackface. One was a white guy playing Star Trek: The Next Generation's Geordi La Forge, and the other was a woman and her friends posing as The Walking Dead's Michonne and her zombie entourage.
Lots of words have been written about this, but it's really pretty simple. This isn't okay. You know why it's not okay? Because the actor's race isn't part of the character's costume. I'm not telling anyone to not dress up as their favorite character, just because their favorite character happens to be a different race. But neither of these costumes needed the blackface to "work." Everybody would know who the Star Trek guy was cosplaying as, based on the visor over his eyes. And Michonne is a pretty recognizable character now, too.
The blackface totally overrides everything else about these costumes. This is basic math. He's not wearing a Geordi La Forge costume. He's wearing a blackface Geordi La Forge costume. If the guy didn't wear blackface, he'd just be wearing a Geordi La Forge costume. Blackface anything is a bad idea. You just shouldn't do it. Period.