Despite what your mother told you, the dark color of a blackhead is not dirt but the normal melanin pigment of the skin. Normally, as the skin renews itself, the top plug falls off and the whitehead or blackhead disappears. However, in some people a bacterium called corynebacterium acnes proliferates in the comedone converts the sebum to more irritating substances that lead to inflammation. Acne thus results from a series of steps, beginning with the action of androgens on the sebaceous glands, causing them to produce more oil. Second, the top of the skin is too sticky and the opening of the pore is sealed off.
Bacteria grow, causing the oil to become irritating and increasing the pressure inside the pimple.
Finally it breaks inside the skin, causing a much larger pimple and considerable inflammation. Acne usually involves the skin of the face because this is where the pilosebaceous glands are most sensitive to the oil-stimulating effects of the androgenic hormones. But the skin of the upper chest and back also produces sebum, and some people get acne in these regions. Everyone's skin becomes oilier with puberty, but not everyone has severe acne. So hormones and oil production are not the only factors involved in acne.