dark blue vs blueness

dark blue and blueness both are nouns.

Word	Noun	Adjective	Verb	Adverb
dark blue	Yes	No	No	No
blueness	Yes	No	No	No

As nouns, blueness is a hypernym of dark blue; that is, blueness is a word with a broader meaning than dark blue:

dark blue: a dark shade of blue
blueness: blue color or pigment; resembling the color of the clear sky in the daytime

Other hypernyms of dark blue include blue.

dark blue (noun)	blueness (noun)
a dark shade of blue	blue color or pigment; resembling the color of the clear sky in the daytime