jet black vs inkiness

jet black and inkiness both are nouns.

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

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

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

Other hypernyms of jet black include black, blackness.

jet black (noun)	inkiness (noun)
a very dark black	the quality or state of the achromatic color of least lightness (bearing the least resemblance to white)