This item is autokeep. This means that if you die with it, you will always keep it.
They can be obtained as drops from ankou, cyclopes, shadow warriors and minotaurs, by pickpocketing monkey knife fighters, or by trading with another player. Black knives can be wielded with a shield or off-hand weapon. Black knives are also one of only two ranged weapons able to harm the tormented wraith, the other being black darts.