The Black Spot is enough to send veteran pirate Billy Bones into a fatal downward spiral of a alcoholism. So what is it?
The black spot is literally just that, a black spot. It consists of a round piece of paper with a blackened spot on one side and a message on the other. It represents a condemnation of one pirate by his crew. If given to a captain, it means that his crew has lost faith in his ability to lead is forcing him to step down from his command. In the case of Billy Bones, the message on his black spot is a demand that he appear before his former crew. It is presumed that failure to comply with a black spot could result in the death of the recipient.
In a way, the use of the black spot represents the democratic nature of pirate crews. Anyone can earn the censure of his shipmates and be called to task. Even captains are subject to them. Long John Silver receives a black spot that threatens to depose him. Only his cunning, manipulation, and shrewdness allows him to retain command.