What happens is this. Within the skin there are units composed of a duct, a hair follicle, and a sebaceous gland. The hair follicle passes through a duct to the surface of the skin. Acne develops when the duct gets clogged with cells that shed from the duct wall. This blocks the flow of oil to the surface of the skin.
Pimples emerge when the plug enlarges and pops out of the duct.
The blackness is not dirt but dark skin pigment. Girls have fewer acne problems because they normally only produce only one-tenth as much androgen as boys. Ask your pharmacist about this. Also, washing the affected areas a few times each day decreases the risk of infection and keeps down the oil on the surface of the skin. In addition, some kids find that the problem gets worse when they are stressed, eat certain foods, or use oily shampoos and cosmetics.