The Black Spot is a literary device invented by Robert Louis Stevenson for his novel Treasure Island. In the book, pirates are presented with a "black spot" to officially pronounce a verdict of guilt or judgment.
In the Disney feature film Pirates of the Caribbean: Dead Man's Chest, Captain Jack Sparrow is presented with a "black spot" by Davy Jones as a marker that the Kraken can track; in the film, the black spot appears as a large black boil-like swelling on the palm of his hand. The "black spot" is s mark of death to a pirate.
In an episode of Dr. Who: The Curse of the Black Spot a Siren marks her prey with a black spot on their hand, a mark that means certain death.