How do I ask what may be an offensive question?

I just read in the paper where a black comedian got pulled off the stage for using the N-word, & other offenses. Years ago, I watched Richard Pryor on Johnny Carson. They were talking about why it was ok for Mr Pryor to use the N-word, but it was wrong for Mr. Carson to use it. The main reason seemed to be that Mr. Pryor was black, & Mr. Carson was white. I think the word is offensive no matter who uses it, or what color they are. I've listened to black comedians use the N-word for years. Why is it suddenly wrong for them to use the N-word? Is it because I'm white that I don't understand this?