coal black vs blackness

coal black and blackness both are nouns.

Word	Noun	Adjective	Verb	Adverb
coal black	Yes	No	No	No
blackness	Yes	No	No	No

As nouns, blackness is a hypernym of coal black; that is, blackness is a word with a broader meaning than coal black:

coal black: a very dark black
blackness: the quality or state of the achromatic color of least lightness (bearing the least resemblance to white)

Other hypernyms of coal black include black, inkiness.

coal black (noun)	blackness (noun)
a very dark black	the quality or state of the achromatic color of least lightness (bearing the least resemblance to white)
	total absence of light