Which description fits a blackhead?

A blackhead is classified as an open comedo. This term refers specifically to a type of acne where there is a clogged hair follicle in the skin that remains open to the air on the surface. The dark appearance of a blackhead is due to oxidation of the melanin in the skin, not because it is filled with dirt, as is a common misconception. In contrast, a closed comedo, commonly referred to as a whitehead, occurs when the clogged follicle is closed off from the surface of the skin.

Understanding the characteristics of different types of comedones is essential for accurate diagnosis and treatment in dermatology. Cystic lesions, on the other hand, refer to deeper types of acne that involve inflammation and are typically more severe, while a fissure is defined as a crack or break in the skin, which is unrelated to the formation or classification of comedones.