jet black vs blackness

jet black and blackness both are nouns.

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

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

jet 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 jet black include black, inkiness.

jet 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