When a bro is intoxicated and chatting-up a girl (one of exceptionally bad quality), it is ANOTHER bros responsibility to warn his bro of the bad decision he is about to make.
In this situation the term "blueberry muffins" is subtly used as code for "Get the fuck out of there! ABORT ABORT!".
Bro sees his bro in trouble and subtly codes to him: "DUDE!! Lets go get some BLUEBERRYMUFFINS!"
When a girl wears clothes that are too small, particularly jeans. Tight jeans push up the fat on her hips, which spills over the sides of the pants like the top of a muffin around the wrapper.