Blackheads are a type of comedo. Comedones occur when the pores in the skin become plugged with dead skin cells and an oily, protective substance known as sebum.
The top of the blackhead, which is visible on the surface of the skin, has a dark color.
Normally, hair grows from hair follicles in the pores, and the sebum-producing sebaceous glands lie underneath.
They are a feature of mild acne, but they can appear without other signs of acne being present.