A Black Sludge is a held item that heals Poison-type Pokémon by 1 of their maximum HP every turn. If the holder is not a Poison-type Pokémon, it will lose 1 of its maximum HP every turn instead. It can be crafted, and can also be obtained as a tier 2 special drop. It is also a drop from certain wild Pokémon.
A Black Sludge is also needed to craft a Toxic Orb.