If you're blinded, then your enemies are effectively invisible. Thus you have to pick a square to direct your attack at (guess targets location) and you still suffer a 50% miss chance.
But I'm not sure how you use a spell effect to blind it's central eye. It's power is an antimagic field. How is a spell going to affect that?
And if it can see with that one, then it doesn't have to guess the targets location. I would still give it a 50% miss chance though, since it wouldn't be used to aiming with something other than the eye that's shooting. If you wanted, you might reduce it to a 20% miss chance instead for targets in front of it.