Historically, bats were often associated with darkness and associated with blindness because they are nocturnal creatures and are most active during the night. This misconception was likely reinforced by the fact that some bat species have small eyes, giving them a visually impaired appearance.
However, it is essential to note that this idiom holds inaccurate due to recent scientific understanding of bats' vision. While some bat species may have limited daytime vision due to their adaptation for night activity, most bats have remarkably well-developed senses, including echolocation, which allows them to navigate and find prey in complete darkness.
The phrase "as blind as bat" is gradually falling out of use in certain circles due to its reliance on inaccuracies and potential negative implications. Instead, more accurate and respectful language is being adopted to describe visual impairment or blindness.