It is rare to have a skin ulcer after a sclerotherapy treatment. Skin ulcers can occur from too strong of a sclerosant or injecting the sclerosant too fast. Sometimes the skin ulcer just occurs.
Often the ulcer will heal on it own. Keep it clean with antibacterial soap and water. Your doctor may recommend an antibiotic ointment. If the area is black, your doctor may debride it or resect it.