first of all I have to admit that I had not read that series of books
I am not really a big fan of Stephen King, he has interesting stories, but most of it end in nowhere and you are just sitting there thinking: whaaaaaaaaaaat?
so I have no previous experience with the characters of that story
therefore I don't see any problem with The Gunslinger being black (apparently he is not black in the books)
especially since Idris is playing him
and lets face it: that man is hooooooooooooooooooooooooooooooot