A blood blister forms when a sudden injury—like a hard pinch or crush—breaks tiny blood vessels under your skin without tearing the surface. Because the outer skin stays intact, the blood pools and traps itself under the surface, mixing with clear fluid. This creates a raised bump that acts as a natural cushion to protect the damaged tissue underneath.

The blister will look like a swollen, fluid-filled pocket that changes color over time. It starts out red, but quickly turns deep purple or black as the trapped blood dries out. Because of where it is on the hand, it can feel tender, throb, or cause a lot of discomfort when you try to move your fingers or hold objects.

Most small blisters heal on their own in a week or two, but a massive one like the one in the image needs extra care. You should never pop a blood blister at home, because the unbroken skin keeps germs out and prevents infection. Instead, wash it gently with soap, apply a cold pack to lower the swelling, and cover it loosely with a bandage.

Over time, the trapped blood will dry up and peel away as new skin grows underneath. However, a huge blister can cause painful pressure or get infected. If you notice signs of infection—like spreading redness, red streaks, pus, or a fever—or if the pain is too severe, you should see a doctor right away to treat it safely.