One hypothesis is that in ancient time, various colours were assigned to different directions; black for north, red for south, yellow for east and white for west. As it was located in the north of the Mediterranean Sea for which the ancients believed to be situated in the middle of the earth. So it was called Black Sea meaning a sea in the north.
Another theory associated the name with of the chemical properties of water. The Black Sea's deep waters are much less saline as a result the microalgae concentration is much richer, causing the colour to become darker. The Black Sea has a depth of over 150 meters, and at a certain depth its waters become toxic and poisonous. Therefore, in the deepest layers of its water there are no living things except sulfur bacteria. Metal objects from ships, dead plants, and animal matter that sunk for a long period of time became covered with a black sludge.
It is called the Black sea because it was a Sea of Death. From the perspective of sailors, the sea was black due to severe storms in the winter, during which the water is so dark it looks black. In past, Sailors who caught in a storm usually died because of the absence of islands at which to harbour and the fierceness of the storms.