Who is dumb enough to still wear blackface on Halloween - or ever?

Amazingly, lots of people. This weekend, social media blew up with everyone from frat boys to celebrities darkening their faces for Halloween. The backlash, as expected, was immediate.

Julianne Hough, a former "Dancing with the Stars" cast member, wore an orange prison jumpsuit and darkened her skin as part of a Halloween costume meant to look like a character from the show "Orange Is The New Black."

And then she tweeted a photo of herself. Within minutes, Twitter users quickly condemned her choice.

"No, Julianne Hough. Just NO. I can't believe 'blackface is not okay' is a conversation we still need to have," one user Tweeted at her.

Hough apologized Saturday, writing: "I am a huge fan of the show Orange is the New Black, actress Uzo Aduba, and the character she has created. It certainly was never my intention to be disrespectful or demeaning to anyone in any way. I realize my costume hurt and offended people and I truly apologize."

Two twenty-somethings in Florida also became Internet fodder this weekend after posting a photo to Facebook dressed at Trayvon Martin, in blackface, and George Zimmerman.

Blackface is a complex part of America's social, racial and pop-culture history, said Robert Thompson, a professor of media at Syracuse University.

"But none of these people are trying to elucidate the historical intricacies. It's common knowledge that this something you don't do," he said. "It's such an obvious thing not to do. It's like walking into an office and calling the first woman you see 'sweetheart.'"

Hough isn't the first celebrity who should have known better. Ted Danson and Billy Crystal, among others, have previously apologized for wearing blackface.