Blue, blue, my love is blue

Our clinic is located near the Black Sea.
I wonder why it is Black Sea and not Blue Sea?

Blue sounds positive:
Blue Danube
Blue jeans
Rhapsody in blue
Blue Peter

Black sounds negative:
Black sheep
Black cat
A black day
The black death

Films like Black Water (2007) do not contribute to a better image either.

'A terrifying tale of survival in the mangrove swamps.'

However, I am sure the sea will be quite happy with its name after it has seen this mould.