A blister is a bubble of fluid that forms under the skin, creating a raised, soft pocket you can usually see and feel. Most blisters are filled with clear, watery fluid called serum that leaks in from surrounding tissue as a response to injury. They range from tiny dots a few millimeters across to large, swollen patches over an inch wide. What a blister looks like depends entirely on what caused it, so knowing the visual differences helps you figure out what you’re dealing with.
Common Friction Blisters
The blisters most people picture are friction blisters, the kind you get from new shoes, raking without gloves, or repetitive rubbing during exercise. These appear as a raised, rounded bubble filled with clear fluid. The skin over the top is usually smooth and intact, and the area around it may look slightly pink or irritated. Small blisters (under about 1 centimeter, roughly the width of a pencil eraser) are called vesicles. Larger ones, over that size, are called bullae.
When a friction blister pops on its own, you’ll see a flap of loose skin over a raw, pink base that may weep clear fluid for a day or two. As it heals, the exposed skin dries and a thin crust forms before new skin grows underneath.
Blood Blisters
Blood blisters look like regular blisters in shape and size, but instead of clear fluid, they’re filled with blood. They start out a light red color and darken over time to purple or nearly black. This happens because the injury that caused them was forceful enough to damage tiny blood vessels beneath the skin’s surface, trapping blood in the pocket rather than serum. You’ll most often see them on fingers, toes, or the palm of the hand after a pinch or sudden impact.
The dark color can look alarming, but it’s simply trapped blood. As the blister heals over a week or two, the color gradually fades and the fluid reabsorbs.
Burn Blisters
Blisters from burns have a different look. A second-degree burn produces blisters on skin that appears deep red to dark brown, with a shiny, moist surface. The surrounding area is typically swollen, and you may notice layers of skin starting to peel. Unlike a friction blister that sits on otherwise normal skin, a burn blister is surrounded by angry, discolored tissue. The blister itself can be large and irregularly shaped, following the outline of whatever caused the burn.
The swelling and discoloration around a burn blister are signs your immune system is actively working to heal the damaged tissue. These blisters take longer to resolve than friction blisters and carry a higher risk of infection because the underlying skin damage is more severe.
Blisters From Viral Infections
Some infections produce blisters with very distinct visual patterns. Shingles, caused by the same virus as chickenpox, creates clusters of small blisters that appear in a band or strip along one side of the body. The rash most commonly shows up on the torso or face and almost never crosses the body’s midline. New blisters continue forming over three to five days, then progressively dry out and scab over. The skin underneath is often red and inflamed, and the area is intensely painful even before the blisters appear.
Cold sores (oral herpes) produce a tight cluster of tiny, fluid-filled blisters on or near the lips. They tend to tingle or burn before becoming visible, then crust over within a week. Chickenpox blisters, by contrast, are scattered across the whole body in different stages of development, so you’ll see fresh blisters alongside older ones that have already scabbed.
Blisters From Skin Conditions
Certain autoimmune conditions cause blisters that look and behave differently from injury-related ones. The key visual distinction is whether the blister feels firm or floppy. Tense blisters, the kind that feel tight and hold their shape when you press near them, are characteristic of a condition called bullous pemphigoid. These typically form on normal-looking or slightly reddened skin and can appear on the arms, legs, or torso.
Flaccid blisters, which are soft and collapse easily, suggest a different condition called pemphigus vulgaris. These tend to rupture quickly, so what you actually see are raw, eroded patches of skin rather than intact blisters. Both conditions produce blisters that appear without any obvious injury, which is what sets them apart from friction or burn blisters. If you’re developing unexplained blisters repeatedly, that distinction between firm and floppy is one of the first things a dermatologist will assess.
Signs a Blister Is Infected
An uninfected blister contains clear or blood-tinged fluid and gradually improves on its own. An infected blister looks and feels noticeably different. The fluid inside turns cloudy, yellow, or greenish as pus replaces the normal serum. The skin surrounding the blister becomes increasingly red, warm, and swollen rather than calming down over time. You may also notice a foul smell, increased pain, or skin around the blister starting to peel or develop small holes.
The most urgent warning sign is a red streak extending away from the blister, moving up your arm or leg. This indicates the infection is spreading along the lymphatic system and needs emergency treatment. A blister that’s simply getting redder and more painful over two to three days, rather than improving, is worth getting evaluated even without that red streak.