I want to point out that this isn't the first time a white Stephen King character has been played by a black actor. That would be the character of "Red" in "The Shawshank Redemption", who actually was Irish in the original novella "Rita Hayworth and the Shawshank Redemption", rather than it being a joking response to where his nickname came from, but was played by Morgan Freeman in the movie.
Why am I pointing this out? Because I don't think anyone would argue that Morgan Freeman didn't absolutely nail the role. I see no reason why Idris Elba can't do the same. As for Matthew McConaughey, I haven't seen much of his acting, and he seems to be a bit of an average Joe, with not much remarkable about him in terms of appearance. In other words, just about perfect for Randall Flagg.
The whole "starting in media res"? That worries me quite a bit, and Stephen King's assurances that it will all be fine aren't really reassuring to me. This is the same guy who has said that he pretty much gives a free reign to directors and scriptwriters, saying that if an author didn't want his or her work changed, he or she should not agree to have their work adapted. Not that I disagree with him on principle, but that isn't the type of "hands-off" attitude that demonstrates any real concern for his work being adapted. Unless you try to use his name for a piece of crap like "Children of the Corn". Then he'll care.