A blood blister is often described as “a dark red or purple bubble under the skin” that can appear suddenly. It forms when tiny blood vessels break but the skin stays intact, trapping blood underneath. These blisters are darker than regular ones and commonly show up on fingers, palms, toes, soles of the feet, or inside the mouth.

Most blood blisters are caused by pinching or repeated friction, such as slamming a finger in a door, tight shoes, or using tools without protection. In the mouth, they may come from accidental biting or dental work. Some people develop them more easily, especially those with fragile skin, circulation problems, diabetes, or bleeding disorders.

Typical symptoms include a raised, fluid-filled spot, dark red or purple color, and tenderness or pressure pain. “Most blood blisters are not dangerous and heal on their own within one to two weeks,” without special treatment.

Should you pop one? “In most cases, no.” The skin acts as a natural barrier against infection. If it breaks on its own, gently clean it, apply antiseptic, and cover it with a sterile bandage.

Simple care includes reducing pressure, using a cold compress, and keeping the area clean and protected. See a doctor if it’s very painful, shows signs of infection, appears often without cause, or doesn’t heal after two weeks.