A blood blister and a hematoma are both collections of blood outside blood vessels, but they differ in location, size, and severity. A blood blister is specifically blood trapped beneath the top layers of skin (epidermis), typically forming a small, raised, fluid-filled bubble usually less than 1 centimeter in diameter. Blood blisters commonly occur from pinching injuries, friction, or burns and are generally minor injuries that resolve on their own within 1–2 weeks.
A hematoma is a broader term for any collection of blood that pools outside a blood vessel, and it can occur in various locations throughout the body—under the skin, in muscles, around joints, or even in organs. Hematomas can range from tiny bruises (petechiae) to large, serious accumulations requiring medical intervention. While small hematomas under the skin appear similar to bruises, larger ones can cause significant swelling, discoloration, and pain.
In practical terms, you might think of a blood blister as a specific small type of skin hematoma. Blood blisters are usually painless or mildly uncomfortable and don't require treatment beyond protecting them from further injury. Larger hematomas, by contrast, may cause considerable pain, require drainage or surgery, and can indicate more serious underlying trauma or bleeding disorders. The key distinction is that "blood blister" refers to a particular small injury confined to skin layers, while "hematoma" is the medical umbrella term for blood pooling anywhere in the body.