I know that Randall Flagg being "the man in black" doesn't mean he is black, but I did feel like it was at best ambiguous in the book, or if he was white he was darker skinned. I could be forgetting something that contradicts this though. Apparently he was originally based on Patty Hearst's kidnapper, who was black (something I just learned myself a second ago). Just an interesting thing to think about with the news of this casting, since I remember thinking about it a lot when I was reading the book, especially with the duality with Mother Abigail. Anyways, I'm excited to see what comes of this adaptation since I love The Stand and have already seen it adapted well in the comic (I never saw the old miniseries).
Also, this is somewhat unrelated, but I think his turn in Big Little Lies was really overrated, and I was kind of surprised at people falling all over themselves for what I took as a pretty typical performance of a horrible abusive husband.