Gangrene and blisters do not smell if there is no open wound and the content inside the gangrene blister or the skin blister is not leaking or exposed. That is why grandma would get a needle and prick the blister to get the content out. Some blisters contain dead T cells that had courageously lost their life fighting off infections. These blisters do not smell.
Sometimes gangrene sores look dry. That is because the clostridia is inside the dying tissue under the dead green skin and the gangrene blister is covered with a layer of skin and the blister containing the foul pus is relatively well protected by the dead skin.