Nah...Unfinished business doesnt alter spells and such...but i got my answer anyway:
Its not broken. The enemy is just blind like the name of the spell says.
His vision radius is reduced to literally one pixel size around him. (You can actually see just how tiny this vision radius is if you blind a party member, or dust mephits in BG2 can also blind you.) If you stand outside his tiny vision radius and attack, he won't be able to see you and won't fight back.
I guess one thing that could be said to be broken would be the pathetic AI in the game. Even a person is blinded, he is still going to realize when an arrow pierces his stomach so he should try to get away. Instead, the pathetic AI of the game merely makes the enemy stand in place like an idiot while you turn him into a pincushion. That is broken, but the spell itself really is not.
Cant Blame a mage for using his avaiable spells anyway....even the strong doomsayer could get blinded eheheheh