(as) blind as a bat

1. Unable to see at all. After neglecting his vision for years, my grandfather became blind as a bat, unable to even see shadows. A: "Didn't Emily get hit by a car and end up blind as a bat?" B: "Yeah, so she could leave town to search for a cure—typical soap opera story. They had to write her out somehow." Doc, I'll do anything—surgery, drugs, you name it! Anything to keep from becoming blind as a bat.

2. Having poor vision. I've worn glasses for 20 years, so I'm as blind as a bat now. Because I'm blind as a bat, I always knew there was a good chance my daughter would need glasses too. That's not a cat, it's a trashcan! Geez, you really are blind as a bat!

3. Oblivious to something. Are you blind as a bat? Those two have been flirting all afternoon! You must be as blind as a bat if you don't see the problem with this plan. Come on, I'm not blind as a bat. I know you two have been scheming behind my back this whole time.

as blind as a bat

having very bad eyesight. informal

This expression probably arose from the bat's nocturnal habits and its disorientated flutterings if disturbed by day. The poor eyesight of bats (and less frequently, moles) has been proverbial since the late 16th century.