Boxers and their big chests sometimes makes it difficult to find a harness that fits nice and doesn't rub. If your dog is pulling and that is what is causing the rubbing, then I recommend trying the Gentle Leader Harness as it discourages pulling. Of course this is just a management tool, and I absolutley recommend training your dog to walk nicely on a loose leash.
Now, if the blisters are caused by the harness itself and not your dog pulling, then I would recommend the Lupine harness. It has straps that form a Y in the front, a single strap that goes down the underside of the dog--nothing in the armpits, and one that circles the midsection. It fits my guys nicely and I've never had any rubbing. Not to mention that Lupine offers a lifetime guarantee for everything "even if chewed"!
Now, if your dog is well behaved and walks nice on a leash, I would recommend the Comfortflex Sport Harness. I use this for my dogs and my female runs flyball in it. It only has two straps that are extremely well padded--one to go across the chest and another that circles the midsection. It evenly distributes any pressure and doesn't cause rubbing. It is, however, not good for dogs that fight the leash, twist, or will try to back out of thier harnesses. If you have a dog that's bent on getting out of it, they will. But for a dog that is well-trained on leash, I haven't found anything better.