Suggest Treatment For Pigmentation And Itching Sensation On Neck

black and itching on neck. Used Lobate GM . What should I do?
i have skin problem on back side ...it is of black color and i feel itching and some time irritation. and it is from last 2-3 year, it started from a small black portion and now it cover the area of 2-3 cm, i also got relief for some days during this period, but temporary for 2-3 days...