Blood Blisters Explained: Causes, Healing Stages, and When to Worry

A blood blister develops when pressure or a shearing force damages tiny blood vessels (capillaries) in the dermis—the layer beneath the outer skin—while the epidermis remains unbroken.

Pressure or trauma presses the skin against bone or another hard surface
Small capillaries rupture and begin leaking blood
That blood becomes trapped under the intact surface layer of skin
The body walls it off, creating a protective pocket

Because blood is thicker and darker than the clear fluid found in ordinary blisters (serum), a blood blister typically looks deep red, purple, or even black rather than transparent.

Blood blister vs. regular blister vs. bruise

Knowing the difference can prevent unnecessary worry:

Blood blister

• Blood trapped beneath intact skin
• Raised, bubble-like appearance
• Often more painful due to pressure
• Heals as the body gradually reabsorbs the blood

Friction (clear) blister

• Filled with clear fluid
• Usually caused by rubbing
• Typically lighter in color

Bruise

• Blood spreads under the skin
• Flat, not raised
• Changes color over time (purple → green → yellow)

A blood blister is essentially a bruise and a blister at the same time—except the blood is contained in one small, sealed pocket.