That's because he's not black, when Roland first enters the mind of Odetta/Detta her first thought was that a white man had entered her mind.

Funny thing is though Idris as Roland is the best part of the movie, they tooks bits from various books in the series and crammed it all into 90-some minutes, this wasn't made by fans of the book. Frank Darabont should have made it, but King being a moron turned down selling the rights to him. So we're stuck with this.

If you've never read the books, I guess the movie is a okay, but I've read them, and saw the film and it's awful. I saw Matthew McConaughey in an interview, you could tell he didn't give a shit about the movie, he spent about 1 min talking about it, and it was obvious he had no clue who the character he was playing, he was probably told to just play a bad guy and didn't read the books.