Nominally blue, it finishes up in the Black Sea (6)
I believe the answer is:
danube
This clue could be a double definition.
'nominally blue' is the first definition.
The answer and definition can be both natural objects as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'it finishes up in the black sea' is the second definition.
The definition suggests a singular noun which matches the answer.
Can you help me to learn more?
(Other definitions for danube that I've seen before include "Vienna's river" , "Major European river" , "South European river" , "Europe's musically blue river" , "River flowing through Belgrade, Budapest and Vienna" .)