Another word for blood build-up is a hematoma. A hematoma is beneath the skin, and usually is visible since there is a mark. Hematoma appears as a bruise and can cause localized pain.  Blood can be build-up due to post- surgery or injury. The hematoma usually is a blood clotted in ac certain space or tissue due to the break in the wall of a blood vessel. Subdural Hematomas are the most common for those having major brain injuries causing the area to be filled with blood.