Are you sure it is BLACK? I checked US Navy, International, and Maritime flags from WWII, and there is no solid black that I could find.
Is it a flag or a pennant? Pennants are triangle shaped (like baseball pennants), but don't go to a full pint at the narrow end. Flags are square or rectangular (like US Flag).
There is a solid dark blue square, which was used by convoys to indicate course alteration. They were used along with pennants that indicated the actual course correction.