Blackheads are a mild form of acne that appear as unsightly, open pores that appear darker than the skin surrounding them. They get their dark appearance from a skin pigment called melanin, which oxidizes and turns black when it's exposed to the air. Blackheads aren't caused by dirt, but by sebum (oil) and dead skin cells blocking the pore. If the pore remains open, it becomes a blackhead; if it's completely blocked and closed, it turns into a whitehead.
Acne (acne vulgaris, common acne) is a disease of the hair follicles of the face, chest, and back that affects almost all teenagers during puberty -- the only exception being members of a few primitive Neolithic tribes living in isolation. It is not caused by bacteria, although bacteria play a role in its development. It is not unusual for some women to develop acne in their mid- to late-20s.