What Causes Itchy Black Patch On Sole And Palm?

Hi, I am a 60yr old male, 5'5" weighing 68kgs. I have a dark itchy patch on my right sole for the last 3-4 yrs and now I have a similar patch on my right palm. On medication(AF 150) the itching stops and the patch on the palm disappears. However that on the sole remains without any significant change. Have cosulted a lot of docs, in vain. Please advise.