The phrase "black sludge" refers to a thick, dark, and sticky substance that resembles thick mud or oil. It is often used to describe something dirty, unpleasant, or toxic. Full definition
A passenger aboard a Long Island Rail Road train out of Penn Station went into the bathroom to wash her hands — only to have a stream of black sludge come out of the faucet. (nystateofpolitics.com)
In one place we moved into, the bathtub was filled with black sludge. (redeeminggod.com)
The batter looks like thick black sludge, but once each piece of dough is rolled in Swedish peal sugar, it looks like a little snow ball. (lapechefraiche.com)