If the blood blister isn't too painful, try to keep it intact, says Mayo Clinic. Cover the blister with a bandage, and seek medical attention if it is painful or if it prevents you from walking or using your hands.
Drain the fluid while keeping the skin intact, says Mayo Clinic. First, wash your hands and the blister. Then, swab it with iodine, sterilize a clean, sharp needle, and use the needle to puncture the blister. Lastly, apply an ointment such as Vaseline, and cover the blister with a bandage. Remember to change the dressing every day, and apply more ointment.