If you cut a fresh piece of aloe vera and rub the sap on the skin, your skin will immediately feel tight. The sap also stops light bleeding. Aloe vera sap may actually work better than first aid cream for external wounds.
If there is a bleeding gangrene blister, applying aloe vera sap on the blister should stop the bleeding and act like first aid cream. The aloe vera sap should also tighten the skin surrounding the blister and help close the open wound. Hopefully, its antibiotic properties will somehow control the growth of the clostridia bacteria.
The purpose of applying aloe vera sap on a gangrene blister is to dry out the blister and tighten the surrounding skin.
There may be other ways to dry out the gangrene blister. Perhaps one can try pouring baking soda on the blister directly to see what happens. The baking soda should react with the moisture of the pus of the blister and produce some oxygen inside the blister. If this happens, the oxygen will kill the clostridia inside the blister. Also, the baking soda should get rid of the foul smell of the pus of the gangrene blister.