Blood Blister vs Melanoma: How to Tell Them Apart

A blood blister (subcorneal or subepidermal hematoma) is a pocket of trapped blood beneath the top layer of skin. It forms when small blood vessels rupture under pressure or friction — pinching, repeated rubbing from shoes, sports impacts, or work tools — and the blood collects in a contained space.

Fresh blood blisters look bright red or purple. Over days they darken to deep purple, brown, or near-black as the trapped blood deoxygenates. Most are 2-10mm. They are typically on hands, fingers, feet, toes, and other areas exposed to friction or pressure.

A blood blister has a clear life cycle. It forms in minutes to hours, peaks within a day, and then resolves over 1-2 weeks as the body reabsorbs the blood. The skin above usually peels off, revealing pink new skin underneath.

The mimic is usually nodular melanoma or acral melanoma. A small dark papule on a finger, palm, sole, or under a nail can look exactly like a friction blood blister. Sometimes melanoma even develops at a site of recent minor trauma, which reinforces the mistaken explanation ('I must have pinched it').

The key difference is behaviour over time. Melanoma does not resolve. It persists, often grows, and may change colour, shape, or texture over weeks. A blood blister that is still there four weeks later is no longer a blood blister.

Blood blister: appears suddenly after a clear injury or repetitive friction. You usually remember the moment — pinched a finger in a drawer, new shoes that rubbed, hammered your thumb. The lesion is at the exact site of the trauma.

Melanoma: appears without obvious cause, or at a site where the 'cause' is vague ('I must have bumped it sometime'). The vague-cause explanation is itself a flag — your brain is constructing a story to fit a lesion that does not have an obvious origin.

Blood blister: peaks within hours, then steadily resolves. By day 7, the dark colour is fading at the edges. By day 14, the lesion is significantly smaller or gone. By day 21, it should be fully resolved.

Melanoma: stable or growing at 4 weeks. The colour does not fade. The shape may evolve (darken, become irregular, gain new colours). If your 'blood blister' is the same or worse at 3 weeks, it is not a blood blister.

Blood blister: typically round or oval, with a smooth, well-defined edge. The colour is uniform — all the trapped blood is the same age, so it is all the same shade.

Melanoma: irregular border is common (the B in ABCDE). Multiple shades within a single lesion (the C in ABCDE) — black, brown, red, blue, even small areas of skin tone. Asymmetric — one half does not mirror the other.

For lesions on fingers, palms, soles, or under nails specifically, look for the parallel ridge pattern dermatologists describe — pigment that follows the natural ridges of the skin in straight, parallel lines, often with a streaking appearance. This pattern is a strong acral melanoma sign.

Blood blister: gentle pressure on the lesion may slightly compress it, and you may see a small clear or bloody fluid pocket. The colour may shift slightly because you are moving fluid around. The skin above is loose and lifted.

Melanoma: firm to the touch. Pressing does not compress it. The pigment is in the cells themselves, not in a fluid pocket. The lesion feels solid against the underlying tissue.

This check is gentle — do not pierce or squeeze the lesion.

If your dark spot is on or under a nail, look at the cuticle and surrounding skin. A blood blister under a nail (subungual hematoma) stays under the nail. The cuticle and surrounding skin are normal.

A subungual melanoma can extend pigment beyond the nail onto the cuticle, nail fold, or surrounding skin. This is the Hutchinson sign and is highly suggestive of melanoma. It requires urgent dermatology assessment — within days, not weeks.

Reasonable to wait 2-3 weeks if: clear injury caused it; smooth round shape; uniform colour; lesion is on a high-friction site; you are otherwise low-risk. Photograph it with a date, recheck at 14 days. By 21 days it should be largely or completely gone.

Book a dermatologist within 1-2 weeks if: no injury you can recall; lesion is on the palm, sole, or under a nail in someone over 30; irregular border or multiple colours; lesion has not resolved at 3-4 weeks; lesion is growing.

Urgent (within days) if: pigment extends from under the nail onto the surrounding skin (Hutchinson sign); rapid growth; bleeding without injury; you have prior melanoma history.