Yes, BLUEST is a Scrabble word!

Letter values in Scrabble US: B 3, L 1, U 1, E 1, S 1, T 1.

Is it a valid word?

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

of the color intermediate between green and violet; having a color similar to that of a clear unclouded sky
used to signify the Union forces in the American Civil War (who wore blue uniforms)
filled with melancholy and despondency
characterized by profanity or cursing
suggestive of sexual impropriety
belonging to or characteristic of the nobility or aristocracy
morally rigorous and strict
causing dejection

Also: down in the mouth, low-spirited, blue-blooded.

Not playable in Scrabble US: ARISTOCRATICAL.