As kids, BlackJack was much larger than SpongeBob. However, when he met SpongeBob again, he was only the size of SpongeBob's big toe. It is likely that he stopped growing when he was young, as evident by SpongeBob saying that he hasn't changed a bit. BlackJack has a very gruffly voice and is circle-shaped, compared to SpongeBob's square shape. He also has very muscular arms for his small size. On his arms are blue-colored anchor tattoos. He has shiny black hair and dresses up in biker-gang clothing in that he wears a white shirt, red wristbands, blue jeans with a belt buckle like SpongeBob's, and black cowboy-like boots. He has a pet worm and lives in a cabin-like house only next door to his Uncle Blue SquarePants' house. BlackJack is known to be very tough.
BlackJack went to prison for littering, but was released from prison during the events of "BlackJack."
SpongeBob receives a note from his cousin BlackJack, saying that he is going to pay SpongeBob's parents a visit. When SpongeBob goes to his parents' house, he another note from BlackJack, saying he is going to pay SpongeBob's grandma a visit.
SpongeBob later goes to BlackJack's house and hears his mother screaming from the basement. He goes down there and it is then revealed that SpongeBob's parents are in no danger; they were only holding a surprise party to celebrate BlackJack's release from prison, as he renounced his criminal ways of littering.
BlackJack still has the intent of beating up SpongeBob however, who is ready to stand up to himself. As he charges at him, it is revealed that BlackJack is no bigger than SpongeBob's foot and can't do any harm whatsoever.
Cousin BlackJack is older and tougher than SpongeBob. He used to be a bully to him and both of them had a sibling rivalry with each other. BlackJack also calls SpongeBob a "little man," not just because SpongeBob is younger than him, but due to the fact that SpongeBob used to not make any attempt to duel with him and used to become scared stiff.
BlackJack is known to be a powerhouse. When SpongeBob told Gary the Snail of BlackJack's release from jail, the snail could be seen scared, meaning Gary had seen or was aware of how dangerous the latter was. SpongeBob's parents were not scared of him at all, in fact, they found him fun that they played and chatted with their nephew. On the other hand, Captain Blue SquarePants was very scared to learn that BlackJack was out of prison (even though they are neighbors) and quick to flee to his home after dropping off SpongeBob.
It is unknown how or why BlackJack has stopped growing.