The blinded condition means that targets have total concealment against the attack (normally a -5 to the attack roll). For an auto-hit power, that doesn't matter - the Magic Missile hits.
From a flavor perspective, remember that blinded doesn't necessarily mean you don't know where the enemy is. You can't see them, but you can still hear them. If they move away stealthily (and you do have -10 to Perception checks while blinded, so sneaking away is pretty easy) then you could become unaware of them.
I would rule that if a monster blinds you and then sneaks someplace else without you hearing them (their Stealth check versus your Perception check with the -10 blinded penalty) then you couldn't hit them with a Magic Missile because you don't know where they are. But if the bad guy hasn't moved, or they've moved while making so much noise that the blinded character still knows where they are (poor Stealth roll or awesome Perception roll) then the Magic Missile still works.
This is how things work in the rules, too - see page 221 of the Rules Compendium for the details on Targeting What You Can't See. Technically the character can still pick a square and shoot at it in the hopes that the bad guy was there, but feel free to rule that as you see fit as the DM (this is rather silly if the bad guy token is still on the table so that other characters can see it - though you could rule that the blind character's allies can yell instructions about where to aim to hit the bad guy).
