I have a serious question

I can't ask anyone I know and, I don't have the guts to look it up. At school last year everyone was saying it. I think I should know before I go back to school. I've been wondering what it means all summer. Everyone thinks it's so funny, I'm obviously missing out on a joke? Ok. It's the phrase "Once you go black you never go back". Pleas don't make fun of me. I literally just came up with the idea of asking anonymously and to people that probably know. Thank you.