To treat a blood blister, WebMD recommends giving it time to heal, avoiding putting weight on the blister and wearing a moleskin doughnut-shaped pad to protect it. A loose bandage can also be worn over the blister to protect it during the healing process.
For large, painful blisters, draining may be necessary. WebMD recommends cleaning a straight pin or needle with rubbing alcohol and using the end of the needle or pin to puncture the blister's edge. Next, press the fluid inside the blister toward the hole and allow it to drain out.
Once the blister has drained or tears open on its own, wash the entire area with soap and water. Avoid using alcohol or other cleaners. If the flap of skin over the blister is torn, dirty or has pus beneath it, remove it; otherwise, leave it intact. Use an antibiotic ointment on the blister, and apply a clean bandage. Change the bandage when it gets dirty or wet, or once daily. Allow the bandage to remain off at night to let the blister and the skin around it to air dry.
Notably, those with conditions like cancer, heart disease, HIV or diabetes should not drain blisters. Likewise, blisters suspected as being from contagious diseases such as chickenpox should also not be drained, according to WebMD.