Yes, BLACKEST is a Scrabble word!

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

Is it a valid word?

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

being of the achromatic color of maximum darkness; having little or no hue owing to absorption of almost all incident light
of or belonging to a racial group having dark skin especially of sub-Saharan African origin
marked by anger or resentment or hostility
offering little or no hope
stemming from evil characteristics or forces; wicked or dishonorable
(of events) having extremely unfortunate or dire consequences; bringing ruin
(of the face) made black especially as with suffused blood
extremely dark
harshly ironic or sinister
(of intelligence operations) deliberately misleading
distributed or sold illicitly
(used of conduct or character) deserving or bringing disgrace or shame
(of coffee) without cream or sugar
soiled with dirt or soot

Also: pitch-black, pitch-dark, black-market.