Hello, Black rash on the lower limb may be due allergic contact dermatitis, viral infection, autoimmune disorders, varicose veins, etc. You are suggested to keep your skin clean, wash it with mild soap (medicated soap) and water. Cold moist compress twice daily may help. Apply ointment / cream hydrocortisone in the area of the skin affected. Take non sedating oral histamines, once daily for 5 days. Avoid using any chemical cosmetics or perfumed products. Wear loose fitting cotton clothes. Since the rash has enlarged and bigger in size you should see a dermatologist. Get clinically examined by a dermatologist, get blood and skin samples taken to detect the pathology causing the discomfort. Get treated. Neglect may sometimes lead to serious infections may manifest that cause tissue death and necrosis like necrotising fasciitis, scaled skin syndrome, dress syndrome etc. which may be life threatening. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Nupur K., General & Family Physician