Black Dog is a pirate most characterized by his unsettling physical appearance. He is missing two fingers on his left hand and sports a remarkably pale complexion, giving off a sinister demeanor and an unhealthy look. Upon first seeing him, Jim notes that he does not seem particularly "sailorly" or much like a fighter, but Black Dog proves to be a dangerous character, often prefiguring the violence of the pirates.
In the grand scheme of the story of Treasure Island, Black Dog is a minor character who is most significant in two scenes. Black Dog first appears in the book's early chapters. He comes to the Admiral Benbow Inn in search of Billy Bones. When the two encounter one another, Billy attacks Black Dog, causing the latter to flee from the inn in alarm.
However, this is not the last of Black Dog: his presence at Long John Silver's tavern acts as a foreshadowing of Silver's true identity as a pirate. Jim recognizes Black Dog there, but Silver denies knowing such a person. Jim is suspicious, but Silver's charm and appearance strike a strong contrast with the more sordid Black Dog, causing Jim to trust in the one-legged sea cook.