Benvolio is trying (and failing) not to blush at what the newcomer is implying. That is... not what was happening, clearly, they were just goofing off as friends, because as Mercutio just stated with his own words, they are friends. He can't figure out what to say that isn't going to sound ...foolish, and not in a purposeful sort of foolishness, so he stays quiet and lets Mercutio handle this.