What Causes Skin Itching Followed By Black Mark On Skin?

sir for me skin is getting itch i am 24 year old some times at night i feel itching once i rub the skin it cause red color and wound happen later it cures and black mark ,will happen in that area.5 feet and 46kg.from at the age of 16 i have this problem after 6 months or 1 year the mark which was happen by itching it will go.