The phrase “as blind as a bat” describes someone who is extremely visually impaired. It comes from a long-standing misconception about bat vision. Because bats are nocturnal and some species have small eyes, people historically associated them with blindness.
Modern science shows this idiom is inaccurate. Most bats have well-developed senses, including echolocation, which allows them to navigate and hunt in complete darkness. As a result, the phrase is gradually falling out of use in favor of more accurate and respectful language.