[Velusion]

I would have assumed it was just the spell "blindness" and can be resisted via MR?

In my experience anything with a pretty high MR (20+?) is pretty much immune to it. I never have a problem of eye loss when I go toe-to-toe SC vs SC when the opponent has an eye shield.

It seems to be much more effective against troops that can't get a high MR.

Thats my (limited) experience at least...