The saying “blind as a bat” doesn’t actually mean anything, because bats aren’t blind. In fact, all 1,100 known species of bats can see. Most of them have very poor eyesight compared to some other nocturnal creatures, but the point is that they CAN see.
Even though they can just fine, they, like most other animals, have poor night-time vision. This can be a huge burden, as most bats are strictly nocturnal. Imagine having to go out every night in absolute darkness with no method to navigate! To overcome this problem bats developed the technique of echolocation. They emit high frequency sounds which reflect of surfaces. By judging how long the sounds take to return to them, the bats are able to create a pretty accurate mental map of their surroundings. They use this technique at night to locate and track predators, prey and obstacles. Due to the under-developed nature of their vision, bats use echolocation for most purposes. Generally they only use their eyesights for distant objects and where echolocation is not possible.
After learning about this amazing skill of bats, people naturally assumed that bats had developed this technique to compensate for a lack of eyesight. This led to a widely accepted belief that bats are blind. Several years passed before scientists and researchers confirmed that all species of bats have the sense of vision. There are even species of bats that are known to have better vision that humans!