Why is the Black Sea called The "Black" Sea?

One of my students asked me this question, and I haven’t the foggiest.

The ancient Greeks called it Pontos Euxenos meaning “the hostile sea” because of its regular storms and tempests. This euxenos adjective AFAIK can also mean black, which is where the sea got its name from.
The Romans took over the Greek name and called it pontus euxenus. I think the Russian name for it also refers to its hostility and inhospitality instead of any color, but I’m not sure about that.

Actually, Pontus Euxinus means “hospitable sea”.

It has two layers of water of different densitites, the lower of which is very saline, contains hydrogen sulfide, and has little movement. Hence, it has no marine life. WAG: perhaps that is why it is called “black”.

If it’s like most nearly closed bodies of water (e.g. Lake Erie), it will actually get black every few decades from algae during droughts.

From the Encyclopedia Americana