The black knife is a throwing knife that requires a Ranged level of 10 to wield. It is stronger than a steel knife but weaker than a mithril knife. As with all black equipment, this item cannot be made via the Smithing skill. A black knife must either be obtained as a monster drop, or bought from another player. They can be poisoned and can be wielded with a shield or god book for protection.
Throwing knives are among the fastest weapons in the game (along with darts), and are commonly used for PKing or training Ranged.