pitch black vs blackness

pitch black and blackness both are nouns.

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

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

pitch 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 pitch black include black, inkiness.

pitch 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