I'm sorry, but you are completely misreading some of the rules here. Line of Sight is not required to have Line of Effect. They are seperate elements. By your logic, being blind would prevent a character from making any attacks at all - I hope its self-evident how absurd that is, especially since there are specific rules for attacking enemies you can't see.
The truth is that Magic Missile automatically hits the target, even if you are blind. Provided, however, that you declare the appropriate square to attack - if you are blind and the enemy sneaks away from where you think he is, the Magic Missile may fail since you are targeting an empty square.
Now, most groups will allow other non-blinded PCs to point out to the blind PCs where the enemies are, so that will usually be enough. If that bothers you, one would instead need to have the Wizard making Perception checks to hear where an enemy is, and if they fail, force him to guess at whatever location he wanted to attack.
