Blackheads

A blackhead is very similar to a whitehead. Both are clogged hair follicles. The source of the blockage is usually dead skin cells. Both a re the result of the sebum oil trapped with the skin.
The difference between the two is exposure to the air. In a blackhead, the pore is not completely blocked. There is enough of an obstacle to prevent the sebum oil from escaping. But air can circulate and expose the oil to oxygen.
Oxygen will react with most compounds. This process is called oxidation. The oxygen molecule attempts to fill its out shell with electrons by reacting with other molecules. The byproducts of oxidation are often carbon dioxide and water.
The sebum oil will darken due to exposure to oxygen. The white color becomes black, and the result is called a blackhead.