Hi there ~
I understand your concerns. The black areas on your palm which itches only on scratching needs medical attention. Since it is there at times and not at others, it might be related to allergic reaction
to a certain substance that you are exposed to. If you find out what the allergen is by thinking about what it might be and avoid it, you have the solution to your condition. However, if the condition worsens, I believe you need to see a dermatologist
. I hope this helps.
Take care and have a lovely day!