Did you know fantastic help is an anagram of Planet Catfish? This forum is for those of you with pictures of your catfish who are looking for help identifying them. There are many here to help and a firm ID is the first step towards keeping your catfish in the best conditions.
This fish is absolutely matt black in all the body, with no spots at all. As it says in the Cat-elog, Synodontis nigrita has a dark gray background with black spots on the body and fins.
At the size you mention (15 cm), the spots on S. nigrita fade away and the fish becomes uniformly-colored.
But it's difficult to confirm without a picture of the whole fish.