What Causes Recurring Open Wounds On The Surgical Scar?

I had surgery in September and the wound never healed. It turned black and blistered open. It has been cultured for MRSA and staph and both cam back negative. Then in January they did another surgery to get rid of the open wound, it happened again only this time worse. No fever, I feel fine and cultures are still negative. What else could it be or what should I have my doctors check for now?