Final answer:
The phrase 'BLIND AS A BAT' is used as a simile to compare Will's inability to recognize Kate's rudeness with the common misconception that bats cannot see well.
Step-by-step explanation:
The phrase 'BLIND AS A BAT' in the sentence 'Will is so in love, he is as BLIND AS A BAT when it comes to Kate's rude behavior' is an example of a simile. A simile is a figure of speech that compares two different things using the words 'like' or 'as'. In this case, Will's obliviousness to Kate's rudeness is directly compared to the way we commonly describe bats, suggesting that his love makes him unable to see her faults, just as bats are (erroneously) thought to be unable to see.