A blood blister can be startling at first sight—a dark red or purplish bubble beneath the skin that seems to appear out of nowhere. Although they’re generally harmless, blood blisters can be uncomfortable and may sometimes indicate an underlying issue. Knowing what blood blisters are and how to care for them can help you manage them properly and recognize when medical attention might be needed.What Is a Blood Blister?

A blood blister develops when tiny blood vessels (capillaries) under the skin break while the outer layer of skin stays intact. The leaked blood becomes trapped beneath the surface, forming a raised, fluid-filled spot that looks red, maroon, or purple—noticeably darker than a typical clear blister.

Blood blisters most commonly appear on:

Fingers and palms

Toes and soles of the feet

Inside the mouth (cheeks, tongue, or gums)

Common Causes of Blood BlistersMost blood blisters are the results of sudden pinching or repeated friction, such as:

Slamming a finger in a door

Wearing tight or poorly fitting shoes

Using tools or sports equipment without protection

Repetitive motions that create pressure on the skin

Blood blisters in the mouth can result from:

Accidental biting

Dental procedures

Sharp or poorly fitting dental appliances

In some cases, blood blisters may appear more easily in people with: