Walking dandruff is a term used when rabbits have fur mite
s. The rabbit scratches and the skin flakes, and the mites themselves are whitish so they can look like moving flakes of skin. Eventually the rabbit scratches so much you'll start seeing scabs as well. I agree with
@ladysown, it looks like fur mites from the pictures. The black skin/fur area could just be the black tips of the chestnut hair growing back.
I'd take a look inside their ears as well to check for ear mites. I've found that once in a while ear mites and fur mites come along together. Look for ears that are dirty, a little (or a lot) crusty down inside, sensitive (rabbit doesn't want you to touch them), rabbit shaking its head a lot or of course scratching at them.