Why does my dog keep getting blisters?

So my dog (well, boyfriend's dog) is a blue fawn pitbull and she has been getting blood-filled blisters since I've known her which is a little over a year. She even had a blister on the inside of her leg that looked like a tumor! Since my boyfriend couldn't afford to take her to the vet AND he's not computer savvy, I decided to google this problem. The first time I searched for info, I found that most dogs of this breed are allergic to their food. So we switched her from Beneful to NutroMax (beef). This seemed to help for a couple of months but the blisters came back. After about 7 months of her eating the NutroMax, we decided to switch her food AGAIN since it wasn't working the way I wanted it to and she didn't seem to like it anyway. So now we're trying Authority (lamb) and she loves it but there are still small blisters present.

I did a little more research and found out that a trip to the vet would probably be a waste of money since no one saw long lasting results. So until then, poor CoaCoa is gonna be a pimple face.

The blisters are a lot smaller than they used to be.