Accumulation of bacteria living on hair, hand soaps, shaving cream, skin cells, lotion, toothpaste, and phlegm causes the black sludge in your drainage system. The issue with this black muck is that it refuses to go away on its own. It’s not something you can simply wash away with a full-blast faucet. That’s because, beneath the black filth on the sink, there’s a bigger problem lurking beneath the surface that’s more than simply unsightly.