Yes, BLACKNESS is a Scrabble word!

Letter values in Scrabble US: B 3, L 1, A 1, C 3, K 5, N 1, E 1, S 1, S 1.

Is it a valid word?

BLACKNESS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the quality or state of the achromatic color of least lightness (bearing the least resemblance to white)
total absence of light

Also: total darkness, pitch blackness.

Not playable in Scrabble US: LIGHTLESSNESS.