Yes, BLACKFISH is a Scrabble word!

Letter values in Scrabble US: B 3, L 1, A 1, C 3, K 5, F 4, I 1, S 1, H 4.

Is it a valid word?

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

large dark-colored food fish of the Atlantic coast of North America
female salmon that has recently spawned
small dark-colored whale of the Atlantic coast of the United States; the largest male acts as pilot or leader for the school

Also: tautoga onitis, pilot whale, black whale, common blackfish, globicephala melaena.