What part of the bird? I believe it's the base of the tail, than that would be the oil preen gland. I also see new feather growth, the base of the feathers still have their sheath. I'm not sure what the black stuff is.
My guess is the black is just dirt. New feathers can be tender to the touch so they may not preen as much when growing some in. It wouldn't hurt to give it a dust will a pyrethrin based poultry dust just in case it's from external parasites. I usually hold them by the legs with their chests on the ground that I sprinkle the dust on them while hold my breath. I rub it around than let them go.