Opened blister that continually produces pus?

I have a 3 inch by 3 inch blister which popped and opened up on the sole of my foot. The skin is no longer present. I went to the doctor and they put on penicillin, but during the next visits they changed the prescription to soaking salts in order to dry the wound. (domeboro) (attempted to go to a specialist, but they wouldnt take my insurance) It has been 2 weeks and every 3-4 hours the bandages will up with puss. Any idea how much longer this should last or what else I can do. Currently, I soak 3 times a day with domeboro and keep it covered.