What is the cause of a rash over the site of a ~2-3 week old deep bruise? What can I do about it?

Itchy isn't it? A bruise is blood spilled into the skin. If you spill enough it can collect and clot. So a bruise may indeed be a blood clot. If it is in the skin, not the vessel, it is not harmful to your health and is known as a hematoma. They can take a long time, weeks to months, to resolve. Your body removes blood from a bruised site one cell at a time. The yellow and green colors come from the body breaking down the blood products. These same products are irritating to the surrounding skin. Sometimes this leads to itch and sometimes to a bumpy rash that is so itchy you bruise the skin all over again with scratching. Topical steroids may help. Sunlight to the area may help breakdown some of the byproducts of the blood. Otherwise it takes time. Now you know. Calcium deposits are hard like calcium and may be associated with skin breakdown and significant pain.