A person whose vision is very poor is as blind as a bat. Example: "Without his glasses, my dad is as blind as a bat."

Bats are animals whose visions is very poor in the daytime. They use sound to fly in the dark at night. "Hey, watch out!" Reply:"Thanks, I lost my glasses and I am as blind as a bat."

You would probably not tell a person directly that they are as blind as a bat, because it might be rude. Example: "Hey, watch where you drive! What are you, blind as a bat?"