Yes, BLOCKBUSTER is a Scrabble word!

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

Is it a valid word?

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

a large bomb used to demolish extensive areas (as a city block)
an unusually successful hit with widespread popularity and huge sales (especially a movie or play or recording or novel)

Also: smash hit.