blackness vs jet black

blackness and jet black both are nouns.

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

As nouns, jet black is a hyponym of blackness; that is, jet 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)
jet black: a very dark black

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

blackness (noun)	jet 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