A blood blister is a raised, fluid-filled sac that forms when blood vessels beneath the skin are damaged, but the skin surface remains unbroken. Its dark red, purple, or black color comes from the presence of blood collected internally. This occurs when a force injures the deeper layers of skin and the capillaries within, leading to an internal collection of fluid. Though they can be uncomfortable, blood blisters are common and usually heal on their own without complication.
How Blood Blisters Form
The formation of a blood blister begins with a direct, forceful injury to the skin, such as a strong pinch or crush. This blunt force causes trauma that separates the epidermis, the outermost layer of skin, from the underlying tissue layers. The separation creates a pocket, which then rapidly fills with fluid.
Unlike a standard friction blister, which contains clear serum or lymph fluid, a blood blister forms when the capillaries are also ruptured during the injury. The blood leaks out of the damaged vessels and mixes with the other fluid, pooling in this newly created space. The intact skin surface keeps the blood and fluid contained in a protective bubble.
Common triggers for this type of injury are often accidental, like catching a finger in a door or drawer. They can also result from intense friction, such as repeatedly using a tool without protective gloves or wearing ill-fitting shoes that rub against the heel or toes. The characteristic dark hue may initially be red and then darken over several days as the blood cells degrade.
Treating a Blood Blister at Home
The most important rule for managing a blood blister is to leave it intact, resisting the urge to pop or lance it. The dome of skin covering the blister acts as a barrier that protects the underlying wound from bacteria and infection. Breaking this barrier unnecessarily exposes the deeper, tender skin to the outside environment, significantly increasing the risk of introducing pathogens.
If the blister is painful or swollen immediately after the injury, a cold compress wrapped in a cloth can be applied for short intervals to help reduce inflammation. Keep the entire area clean by gently washing it with mild soap and water, taking care not to scrub or apply heavy pressure to the blister. After cleaning, the blister should be covered with a sterile, non-stick dressing or gauze pad to protect it from further friction and accidental rupture.
Allowing the blister to heal naturally means the fluid will gradually be reabsorbed by the body, and the protective skin layer will eventually dry and peel away. Healing typically takes about one to two weeks, as new skin cells form beneath the protective bubble. Avoiding pressure on the area, such as wearing open-toed shoes for a blister on the foot, is also helpful for promoting undisturbed healing.
When to Seek Medical Attention
Most blood blisters are minor injuries that resolve without professional intervention, but certain signs suggest a need for medical evaluation. Any indication of a secondary infection warrants a visit to a healthcare provider. These signs include:
- Increasing pain
- Spreading redness
- Warmth radiating from the site
- Discharge of yellow, green, or white pus
A fever accompanying the blister is a serious symptom that indicates a systemic response to a potential infection. A blister that is large (over an inch across) or one that makes it difficult to use a hand or walk due to pain should be checked. Blisters that appear in unusual or sensitive locations, such as inside the mouth, near the eye, or on the genitals, also require prompt professional assessment.
If the injury that caused the blister was severe, like a crush injury involving a joint, bone, or tendon, medical attention is necessary regardless of the blister’s appearance. If a blood blister appears without a clear cause, keeps recurring in the same spot, or does not begin to show signs of healing within two weeks, a doctor should examine it to rule out any underlying condition.