I think its just the start of a new polyp. I added a new devils hand leather recently also and noticed a black spot... after a day or two, it turned into a polyp. That would be my guess anyway.
My devil's hand leather shrinks at nigh and expands after the lights come on. I noticed the same thing on mine right when the lights came on. Once it expanded the black area went away. It looked exactly like your picture. I've noticed it several times and it always goes away. I don't think it's any things to worry about.