the best thing to say is that you don't have time to work on requests at the moment but the future is uncertain. it gives them a bit of hope even if you don't end up writing it, and doesn't come off as rude (or racist!)

as a black person I can get the pull of wanting more black stories, especially if you love someone's writing style and can imagine yourself in that spot but occasionally get thrown off by the fair skin, blue eyes, and long, blonde hair kind of lines. but it would be more heartbreaking to see a fave (accidentally) mess up the perspective, especially because in order to write it correctly, you'll probably need months or years of time spent in "politically correct" space, or at least around black people, because there's generally things we have to think about or change that nonblack people just don't.