Why is the Black Sea called the Black Sea?

The leading explanation is directional. Several ancient peoples, including the Persians and later the Turks, used colors to denote compass directions, with black for north, white for south, red for the south or west, and so on. From Anatolia, the Black Sea lies to the north and the Red Sea to the south, and the names track that scheme. The Greeks originally called it the Inhospitable Sea, reportedly because of its storms and hostile shores, and then diplomatically renamed it the Hospitable Sea once they had colonized its coasts.

Two physical features fit the name well enough to keep the folk explanations alive. The sea is the world's largest meromictic basin, meaning its deep water does not mix with the surface. Below about 150 meters the water is anoxic and saturated with hydrogen sulfide, so nothing but bacteria lives there, and metal objects and organic material that sink into it turn black. That same chemistry preserves shipwrecks extraordinarily well, and ancient vessels with their timbers intact have been found on the bottom.

The other named seas follow different logic. The Red Sea likely refers to seasonal blooms of a reddish algae or to the color scheme above. The Yellow Sea is named for the silt the Yellow River carries into it. The White Sea in Russia is frozen and white for much of the year. And the Dead Sea is named for what its salinity does to life, which is also why it appears on this site under a different question.