What does Mercutio say about blind love?
The entire line (2.1.33) reads: "If love be blind, love cannot hit the mark." Mercutio is being bawdy here. A "mark" was considered a woman who might provide sexual gratification. If one is blinded by higher notions of "love", he might miss a chance for sex.
Further support for this argument is found in the lines that follow: "Now he will sit under a medlar tree/And wish his mistress were that kind of fruit/as maids call medlars when they laugh alone." Shakespeare scholar Brian Gibbons says that proverbially, fruit was "never good until they be rotten," ie, a woman who has surrendered her virginity. Additionally, "medlars" are a type of fruit thought to look like female genitalia.