"Blind as a bat" is a reference to someone not being able to see well. The meaning may be either literal or metaphorical — referring to an actual physical environment or a situation. As an idiom, "blind as a bat" could refer to a person's inability to see another individual or relationship clearly.
The saying has been around for centuries. It is considered to be one of the older English sayings. Many use it in a joking manner to refer to someone who can't see clearly without glasses. As an interesting side fact, bats are not fully blind. While their daytime eyesight is considered to be subpar, they are able to see better at night.
It is during the daytime that most bats have trouble seeing objects clearly. They tend to be nocturnal animals and use their sense of hearing to navigate environments. the expression could be used to describe someone who is clumsy or navigates himself poorly through physical settings.
Those who suddenly experience degeneration in their vision capabilities might describe themselves as being "blind as a bat." It is often used in a sarcastic manner to poke fun at the situation. Some use the expression to express humor about the aging process or make light of having to get prescription glasses.
For example, an individual who wears bifocals may joke about his vision by stating that he is unable to locate items in a room because he is "blind as a bat." Others may poke fun at their ability to see signs or locate landmarks while driving. Some people might use the expression to indicate that they are not very observant of their environments, missing things that are right in front of them.
"Blind as a bat" might also refer to a person's metaphorical blindness when it comes to negative traits and characteristics in others. When someone enters into a friendship or romantic relationship with another person, he may only notice the qualities that he wants to see. Positive or common qualities may "blind" him to the other person's true nature and intent. Others who observe the relationship and can “see” traits from an unbiased perspective may use the saying.
Some people may not see situations clearly. For example, they may have unrealistic expectations when it comes to securing a job after graduation from a degree program with no experience. This is a type of "blindness" that others may describe by using the idiom. Individuals may also misjudge their own capabilities and skills and be unable to "see" themselves clearly.