A black hair on the arm should be a normal phenomenon, because of individual differences, some people’s skin that is to say, in the normal skin may grow one, or a few scattered hair, and this hair sometimes will show a certain thick, and also grow longer and darker. Generally, it is because the local melanocytes are more vigorous, or the local hair follicle formation base is more developed, and the local hair follicle appears to be proliferating or growing. This can occur anywhere on the skin, not only on the arms, but also on the chest, back and even the abdomen.