small pocket of fluid within the upper layers of the skin

A blister is a small pocket of fluid in an upper layer of the skin caused by friction, burning or freezing. Most blisters are filled with a clear fluid called serum or plasma (blister water). Some blisters, however, are filled with blood; they are called blood blisters.

Blister caused by a chemical burn

Blood blisters change

A blood blister is a pocket of fluid filled with blood.