Since the afterbirth DLC you can stumble upon super secret rooms with nothing but spikes and a black poop in the center of the room.
Is there anything more to it or is this just a "You get nothing!" type of thing?
I've never found anything useful in those rooms beyond what may or may not be in the poop. I wondered the same thing myself a while back and looked at a few different BoI wikis and forum posts and nothing indicated there was anything more in there.