Normally, hair grows from hair follicles in the pores, and the sebum-producing sebaceous glands lie underneath.
When these pores are plugged, the dead skin cells in the open pore react with oxygen in the air and turn black, forming a blackhead.
This is often confused with trapped dirt, but the development of blackheads is not related to the cleanliness of the skin.
Other acne lesions are usually closed, but in blackheads, the skin around the clogged pore opens, air gets in causing the collected sebum oil or dead skin cells to oxidize and turn black or sometimes yellowish.
Blackheads appear most frequently on the face, back, neck, chest, arms, and shoulders. There are more hair follicles in these areas.