The term "blackhead" refers to open or closed comedones, which are blockages of the pores by sebum, dead skin cells, and keratinization.
The mechanism: Sebaceous glands located in the skin's pores produce sebum. If this sebum is not properly removed—for example, due to increased sebum production or keratinization of skin cells—a plug forms. This plug can turn black ("blackhead") or appear as a whitish dot. Blackheads look black when the sebum reacts with oxygen on the skin's surface—this oxidation process darkens the plug. It is therefore not dirt, but a natural chemical reaction.
Blackheads are particularly common in the so-called T-zone (i.e., forehead, nose, chin)—where there are more sebaceous glands.