Blackness is complete darkness. Total blackness, the blackness of the cave, blackness fell.
It describes the absence of light rather than a colour on a surface — the blackness of deep water, the blackness of space. In the twentieth century blackness also came to mean Black identity and experience, which is now the sense most often meant in cultural writing. Note black, blacken; and that the compound with -ness is Middle English and unchanged.
1nounblacknessComplete darkness.
“When the candle went out, we were left in total blackness.”
2nounidentityBlack identity and experience.
“Her poems explore blackness and what it meant to grow up in a mostly white town.”
Too few real examples of this word to estimate which sense is commonest.
blackness is darkness or identity.
Middle English blaknesse is black plus -ness. The Old English blaec meant black and blac meant pale, which is a confusing pair that English eventually sorted out.
Middle English
blaknesse
English, today
blackness
Middle English blaknesse, equivalent to black + -ness. Sense 5 most likely both originated from the masque The Masque of Blackness by Ben Jonson and later changed to its current positive sense.
Etymology from Wiktionary, CC BY-SA