The black gunk is basically build up from leftovers, uncleaned stuff, and mostly a collection of hair, dirt, grease, grime, phlegm, toothpaste, lotion, skin cells, shaving cream, hand soaps, etc.
These things cling down the drain pipes and never leave the place, which becomes a convenient place for bacteria growth. Over time, the bacteria creates black layers, and eventually blocks the drainage system.
Well, now, the crux of the matter is this thing doesn’t go away by itself; for example, pouring gallons of water in the drain will do nothing. The clogging remains. Furthermore, what becomes irritating is that water sinking becomes a time killer in the drains, and a pure inconvenience for everyone.