Hello! The ganglion has a clear colourless fluid/ jelly like substance inside it; on getting injured there must have been bleeding inside turning it into a black cyst; further it might have got infected due to breach of intact skin during the injury which is causing pain. A ganglion is effectively treated by a minor surgical procedure - passing SETONs with a thick needle - extracting the jelly-like substance with a to-and-fro motion, leaving behind the thick sutures/ threads which act like a wick to drain the ganglion thus preventing recurrence; an antiseptic dressing should be done during the period to prevent infection.