★ The first reason
The color of the black indicates the north, the red color pointing to the south, the yellow color pointing to the east, and the Black Sea to the north, so it was associated with his location On the compass.★ The third say
It is associated with the color of the Black Sea water, the sea blue is lighter and lighter than the Mediterranean Sea, and this color is due to the large presence of phytoplankton in its waters because it is fresh due to its origin (Danube).
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From @fatenbasheer