The black sea has a very high hydrogen sulfide concentration in the water, and because of this a black sludge forms on things sunk at the bottom of the sea. So, the black sea gets its name from this material that builds up on the sea floor. When things like metal from ships, dead plants, and animal remains sink to the bottom for a long time this sludge forms on the settled pieces. This same concentration in the water makes it impossible for most things to live at the bottom of the black sea, so there’s not a lot of movement to stop this sludge from forming.