Sea water is actually colorless. If you held ocean water in your hands, you would see that readily enough.
But the surface of the sea absorbs all except the blue rays of the sun. These rays are reflected by the water, and make the ocean appear blue.
Minerals and tiny organisms in water make the seas take on different colors in some places. Some seas appear deep blue, while others are blue-green.
The Black Sea, Yellow Sea, Red Sea, and White Sea are all as blue or blue-green as any other’ sea!