Are you doing the soaks and cleaning daily? Is she spending most of her time lying down or resting on a roost bar. Or is she getting up and walking around during the day? Breast blisters which can get infected with pus can be from lying on damp or soiled bedding and from a rough roost bar. There is a skin type of Mareks disease, but it would affect most of the body. I would ask the vet office to give you the results of the feather testing. But make sure that she is on dry bedding or clean towel or puppy pad, and examine roost bars for any roughness and smooth those down. Consider cleaning out the bedding in the coop now that spring is here.