blackness vs coal black

blackness and coal black both are nouns.

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

As nouns, coal black is a hyponym of blackness; that is, coal black is a word with a more specific, narrower meaning than blackness:

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

Other hyponyms of blackness include ebony, jet black, pitch black, sable, soot black.

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