If you were proposing they got blindness by another name, then it would be a bad one as you say.

However, a few things to consider where game balance is concerned.

Could this give too much of a power increase to assassins? I think so.

No scan? Sounds ok. A few things do this already but it is still an interesting effect.

Items in inventory? Perhaps just have an inventory that is listed as item1, item2, item3 etc, and make they guy use "wi item3". If he knows his inventory well enough to know that's his sword, he'd probably not need to see the sword. If item3 is in fact a steak, he'd grab at a steak and achieve nothing.

Basically this would stop people using where, because all others would have to do is go invis. However, what if they showed up on where as "male storm giant" or "drow thief" anyway?

Perhaps another twist might be that too much light (ie glowing items) in the room hurts their weak eyes.

Just some thoughts.