A blood blister looks like a raised bump filled with dark red, maroon, or purplish fluid, sitting just beneath the top layer of skin. Unlike a regular blister, which contains clear fluid, a blood blister gets its distinctive color from blood that has pooled inside after tiny blood vessels were damaged. They range from a few millimeters to roughly a centimeter across and feel tender to the touch.
How Blood Blisters Differ From Regular Blisters
A standard friction blister forms when the outer layer of skin separates from the layers beneath it, and the gap fills with clear, watery fluid. The fluid acts as a cushion, protecting the damaged skin while it heals. A blood blister goes a step further: the force that caused it was strong enough to break tiny blood vessels called capillaries in the deeper skin layers. Blood floods into the pocket instead of clear fluid, giving the blister its dark, bruise-like appearance.
The shape is the same as any blister, a dome-shaped bubble on the skin’s surface, but the color is the giveaway. Fresh blood blisters are bright red to deep crimson. Over the next few days, the trapped blood darkens as it begins to break down, shifting to a deep purple, bluish, or even near-black shade. This color change is normal and follows the same pattern you’d see in a healing bruise.
Where They Commonly Appear
Blood blisters tend to show up in areas that experience sudden pinching, impact, or intense friction. Fingers and hands are especially common, often from catching skin in a door, a tool, or a drawer. Toes and feet develop them from tight or poorly fitting shoes, particularly during long walks or runs. They also appear on the palms, heels, and the balls of the feet, anywhere skin gets squeezed or crushed rather than just rubbed.
Blood Blisters Inside the Mouth
Blood blisters can also form on the soft tissues inside your mouth, most often on the soft palate or the inner cheeks. These oral blood blisters are sometimes triggered by minor trauma like biting your cheek, eating sharp or crunchy food, or even hot drinks. They appear as a dark red or purple raised spot on the pink tissue of the mouth, and you may feel a brief stinging or burning sensation just before one forms.
Most oral blood blisters rupture on their own within minutes to 48 hours, often during a meal, releasing a small amount of blood. The spot they leave behind typically heals within about a week without scarring. Long-term use of inhaled corticosteroids (common in asthma treatment) can thin the mouth’s lining and make these blisters more likely, particularly in older adults. In rare cases, a large blood blister in the back of the throat can swell enough to affect breathing, which requires immediate medical attention.
How They Heal and Change Over Time
Most blood blisters heal on their own within one to two weeks. In the first day or two, the blister is raised and taut, with a deep red color. As healing progresses, the blood inside darkens and the fluid gradually reabsorbs into the body. The blister flattens, and the overlying skin may dry out and peel away, revealing fresh pink skin underneath. The whole process mirrors a bruise fading through shades of purple, brownish, and yellowish before the skin returns to normal.
The best approach is to leave the blister intact. The unbroken skin on top acts as a natural bandage, keeping bacteria out while new skin forms beneath. Popping or draining a blood blister removes that protective barrier and significantly raises the risk of infection. If the blister is in a spot that gets constant pressure, like the sole of your foot, covering it with a soft bandage or blister pad can reduce friction and prevent it from tearing open on its own.
Signs of Infection
An intact blood blister that follows the normal color progression described above is rarely a concern. But if the skin around the blister becomes increasingly red, swollen, or warm, or if you notice pus (cloudy yellow or green fluid) leaking from it, those are signs of infection. Pain that gets worse over the days rather than better, red streaks radiating outward from the blister, or a fever all point to infection that needs medical treatment.
Blood Blisters Under the Nail
A blood blister under a fingernail or toenail, called a subungual hematoma, looks like a dark bruise or smudge trapped beneath the nail plate. It usually appears quickly, within hours of an injury like slamming your finger in a door or stubbing a toe. The trapped blood is visible as a reddish-purple to black spot that stays in place and may gradually grow out with the nail over weeks or months. You might see traces of the dried blood when you eventually trim the nail.
This matters because a subungual hematoma can look similar to subungual melanoma, a rare but serious skin cancer under the nail. There are key visual differences. A hematoma looks like a round or irregular smudge and appears suddenly after a known injury. Melanoma, on the other hand, typically shows up as a dark streak or line running lengthwise along the nail, develops slowly without an obvious injury, and may widen over time. If you notice a dark line under your nail that you can’t trace back to any injury, or if the discoloration doesn’t grow out with the nail over a few months, that warrants a closer look from a dermatologist.
What Causes Them
The vast majority of blood blisters come from a single, identifiable event: a pinch, a crush, or intense friction. Closing a drawer on your finger, wearing stiff new shoes on a long hike, or gripping a tool tightly for hours are classic triggers. The common thread is force applied to the skin that’s severe enough to rupture capillaries beneath the surface but not severe enough to break the skin open.
Certain medications that thin the blood can make blood blisters form more easily, since capillaries break and bleed under less force. People with diabetes or circulation problems may also notice they develop blood blisters more readily and that they take longer to heal. Repeated blood blisters in unusual locations without an obvious cause are worth mentioning to a doctor, as they can occasionally signal a blood clotting issue or other underlying condition.