What causes blackness at the end of a tkr scar?

Hemosiderin: Most common cause of dark spot near a scar or injury is hemosiderin from the blood that occurs during an operation. The red blood cells break down and release their iron (hemosiderin) it is taken up by scavenger cells that stay in the areaand can stain the skin. Another cause is a retained suture if the area is very small. See your surgeon or a dermatologist for evaluation.

Hemosiderin: Often bleeding into the subcutaneous tissues occur at the time of surgery. Blood has a pigment called hemosiderin which is left in the skin when the blood is resorbed. This causes a darken area of skin.