A blood blister is a type of blister

Blister

A blister is a small pocket of fluid within the upper layers of the skin, typically caused by forceful rubbing , burning, freezing, chemical exposure or infection. Most blisters are filled with a clear fluid called serum or plasma...

 that forms when subdermal tissue

Tissue (biology)

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

s and blood vessel

Blood vessel

The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s are damaged without piercing the skin

Skin

-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

. It consists of a pool of lymph

Lymph

Lymph is considered a part of the interstitial fluid, the fluid which lies in the interstices of all body tissues. Interstitial fluid becomes lymph when it enters a lymph capillary...

, blood and other bodily fluids trapped beneath the skin. If punctured, it suppurates a dark red fluid. Sometimes the fluids are cut off from the rest of the body and dry up, leaving behind dead cell material inside the blister with a texture like putty

Putty

Putty is a generic term for a plastic material similar in texture to clay or dough typically used in domestic construction and repair as a sealant or filler. Painter's Putty is typically a linseed oil based product used for filling holes, minor cracks and defacements in wood only...

. Some blood blisters can be extremely painful due to bruising where the blister occurred.
Common areas that suffer from blood blisters include the hands and feet.

When the cause is abrasion, moleskin

Moleskin

Moleskin, originally referring to the short, silky fur of a mole, is heavy cotton fabric, woven and then sheared to create a short soft pile on one side. The word is also used for clothing made from this fabric, as well as adhesive pads stuck to the skin to prevent blisters.Clothing made from...

 can be used to prevent blood blisters from forming.
Blood blisters are commonly caused by accidents in which the skin is pinched by a tool, mechanism, or heavy weight without protective gear.