Black Sludge is a unique item, which increases the healing effects of a Poison-type Pokémon in Pokémon Sword and Shield. If you have a specific Poison-type Pokémon you regularly use in the game, it’s an excellent time to make sure they have this item whenever they rush off into combat.
To find this item, you need to reach the end of the game. You need to make sure you have all eight of the Gym leader badges in your inventory, and you’ve reached the Battle Tower. You don’t need to defeat the Battle Tower to acquire it, but it’s nearby the large establishment.
When you’re in Wyndon city, start at the Pokémon Center and go to the left, and down the straight towards the Battle Tower. Proceed down the street until you reach the bridge. Make sure you don’t cross it. Before you cross the bridge, go to the right pathway
You should see a small flight of stairs on the right pathway, and if you go down them, you don’t have to go much further. After a few paces, you should see a distinct sparkle overtop that looks like a sewer drain. Approach it, and you should find Black Sludge.
You want to make sure you give it to a Poison-type Pokémon because if you don’t, and they only have Poison attacks, they start losing health during combat.