Blackheads and acne in general usually develop after the onset of puberty, when hormone levels surge and reach the skin. Certain hormones overstimulate the sebaceous glands, resulting in the overproduction of sebum.
Sebum is designed to lubricate the hair and skin and usually drains out of the pore opening on the surface. Sometimes the sebum becomes blocked and bacteria begin to accumulate. If the buildup of bacteria and sebum has no opening to the surface of the skin it is called a whitehead. A blackhead forms if the pore opens to the surface allowing air to enter.
Sebum contains melanin, a pigment. It is the same substance that gives skin its color. Melanin turns dark (oxidizes) when exposed to air. That is why blackheads are dark1. The dark color does not mean they are dirty, and scrubbing does not remove them.