Yes, BLINDSIDES is a Scrabble word!

Letter values in Scrabble US: B 3, L 1, I 1, N 1, D 2, S 1, I 1, D 2, E 1, S 1.

Is it a valid word?

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

catch unawares, especially with harmful consequences
attack or hit on or from the side where the attacked person's view is obstructed