is a metabolic disorder in which amyloid material gets deposited in the skin causing blackish discoloration.
Do you have habit of rubbing your skin vigorously while bathing with scrubber or stone?
Do you itch your skin with nails/plastic object?
Stop rubbing of skin.If you have itching use soft cotton clothing to rub.
Treatment consist of topical /oral steroids
can be tried in your case.
Please visit your dermatologist
who can suggest you treatment options.