You don't have to worry as it's definitely not gangrene. Blisters under the foot occur due to friction or heat or moisture. It's common and if it's not painful, it usually gets healed by itself. As in your case it's painful, there are some measures you need take.
- As the blister is painful, you need to drain it. Firstly, clean the area with water and soap properly. Apply butadiene and clean it.
- Take a hypodermic needle, sterilize in hot water for some time. Take the needle and prick at the base on sides and drain the fluid completely. But remember not to peel away the skin covering the blister as this will predispose it to infection.
- Apply povidone ointment on the drained blister. Take cotton and cover the area, to prevent infection. I also suggest to not always keep it closed, but keep it open to air sometimes, will heal faster.
- Wash and apply butadiene every day.
- But, as there are many blisters appearing in your case, you need to see the cause. It could be a new shoe or slipper you are using that is causing friction. Find the cause and prevent new blisters from coming. Try using old socks when using new shoes, the friction will be lesser.