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If held by a Poison-type, the holder recovers 6.25% of their maximum HP at the end of each turn, rounded down. This healing occurs after damage from sandstorm or hail, but before damage from poison, burn, Nightmare, or Leech Seed. If held by a Pokemon that is not a Poison-type, the holder is damaged by 12.5% of their maximum HP at the end of each turn, rounded down. This damage occurs after damage from sandstorm or hail, but before damage from poison, burn, Nightmare, or Leech Seed. A Pokemon with Magic Guard will not be damaged by Black Sludge.
Most of the time, Black Sludge will be inferior to Leftovers. Why is this, I hear you ask? Surely Black Sludge is superior if held by a Poison-type, as not only does it heal the same amount, but it also damages any Pokemon who tries to use Trick? Well, yes, this is true. However, there is a problem with this—even if your opponent does accidentally get Black Sludge by Trick or Switcheroo, your Pokemon is still crippled, while the Trick user can just shrug and Trick your Black Sludge onto a different Pokemon on your team, later in the battle, thus crippling two Pokemon on your team, instead of one. Of course, you could always predict this and switch in your original Poison-type—but this is a gamble. In short, Leftovers is the safer item, even for Poison-types, but if you want to gamble with Black Sludge, they will be functionally identical in nearly all cases. Furthermore, if you are playing with Item Clause on, Black Sludge is a useful item to get essentially two copies of Leftovers (although in most metagames where Item Clause is used, such as VGC, Black Sludge is a poor item choice anyway as your Pokemon is unlikely to be around for long enough to appreciate the healing).
Wild Garbodor (50%).
Wild Trubbish, Croagunk, and Toxicroak (5%).