Blood blister

A blood blister is a type of blister that forms when blood vessels are damaged but the skin is not pierced. They are most commonly caused when skin is pinched by a tool or mechanism, although they can also be formed during combat sports like wrestling. 
A blood blister will heal over time, and should not be pierced. It should be covered to prevent further contact and regularly re-dressed to prevent infection.
Blood blister at WikipediaCategory:Symptoms