Some inarticulate brachiopods made their shells from a phosphatic mineral composition and so their shells are often colored black when preserved. Below is a good example of an Orbiculoidea truncata that I found in the Windom shale. The shell is small, round, and is conical in shape with an offset beak. Concentric growth lines are present though some parts of the shell are missing. This is the pedicle valve which was also the "top" of the brachiopod as the brachial valve would have been cemented to a hard substrate.
This specimen was found at a borrow pit on Deep Springs Road in Madison County, NY which exposes the Windom shale member of
the Moscow formation (middle Devonian in age, Givetian stage.)