The Black Hole Scam was a scam that can no longer be performed anymore because the Black Hole was removed. It involved dropping an item and trapping the unsuspecting players.
The Black Hole was a place where the banned players went. The banned player had to stay in there until their ban lifted. Many users broke the rules to see what it was like in the Black Hole, so Jagex made it that by buying a Disk of Returning, an unbanned player could visit inside of the place, and interact with the banned players. Many of the banned players said that if you drop your disc, something good will happen. When the unbanned dropped their disc, then the banned will pick it up and teleport out of the place, and leaving the unbanned inside of there. They had to wait until Jagex rescued them. It should be noted that teleporting to Lumbridge was not in the game, and if the player had Law runes used to teleport, they could not be used, since all law runes were removed when in the Black Hole.
The disks were left in the game as a discontinued item, and are now sold as rare items for 67,449,676 coins.