What Causes Recurring Itchy Black Spots/boils On Legs?

Hi doctor my age is 33yrs my height 5 ft and weighs 89 kgs. I have aproblem with my legs they have black spots they come in a form of a small boil which has pus and they are ichy and when i scratch the are a becomes black after healing i have gone to different hospitals some tells it is allegy,i have used like hydrocotesene creme,celestimine cream but no change for 10 yrs now they come and disappear ihave gone for HIVtest but the results is negative please help me to know the cause because the same problem is now affecting my right hand.thank