I would check those birds for mites. Feathers shouldn't usually look that dark, dirty and black. Mites can cause feather loss and the appearance of molting. Those areas aren't exactly usual places for a molt to begin, but are common areas for mite infestations.
I tried to get better pics. The specs of dark are bigger chunks and nothing moving. We live in Washington and it's rained a bit so the dirt they are bathing is a bit moist and sticky. We also have another breed and they don't have any issues. Would mites be more at the base of the feathers?