There are numerous causes for a black spot on your leg such as bruising, dermatologic causes, vascular causes, or deep vein thrombosis. Deep vein thromboses are the most dangerous as clots in your leg can break off and cause pulmonary embolisms – a deadly condition. Vascular causes such as varicose veins may be painful but not life threatening. Dermatologic causes such as acanthosis nigricans present in diabetic patients as a dark velvety patch of skin that is asymptomatic and may resolve if the diabetes is treated.
The ultrasound you received rules out the most deadly cause of a dark spot on your leg – deep vein thrombosis. If you are still concerned, you should seek medical attention from your primary care physician.