The Black Spot was invented by Robert Louis Stevenson for his novel Treasure Island. In the novel, the black spot is a circular piece of black paper or card and is placed in a pirate's hand to officially a verdict of guilt or judgment. The spot was feared by pirates for being a sign of execution. In the book, Billy Bones is so terrified that he actually dies just from looking at it.
There are no records of the spot being used in reality, but its presence has been ingraven into pirate lore and pop culture of today. It appears in multiple pirate games and stories, including 'Pirates of the Caribbean' where it appears as a black swelling on someone's hand that is used as a tracker for Davy Jones's Kraken to hunt down.