Roland was obviously white in the books, and it was a very minor plot point that there was racial tension between him and Detta, but that is easy to overcome (just like loosekanen points out). You could take the position of, "why make unnecessary change to the book?" but by the time this hits the screen it will probably be nearly unrecognizable as originating from the books anyway. Roland being black will be the least of the changes, if they aren't even starting off the movie from book one.
As an aside, it has been a few years since I read the books, so I went back to wikipedia and read the plot summaries for all of them. Those summaries really drive home how great the early books were, and how off the rails the series went towards the end.