What is a characteristic of a blackhead?

A blackhead is defined as an open comedone, which means it is a clogged hair follicle that remains open at the surface of the skin. The dark appearance of a blackhead is due to the oxidation of melanin when it is exposed to air, not because it is dirt. This characteristic allows for air to enter the clogged pore, which is a distinguishing feature compared to whiteheads, which are closed comedones that do not allow such exposure. Understanding this distinction is crucial in dermatology, as it helps in the appropriate treatment of various types of acne. The other options do not accurately describe the formation or impact of blackheads on skin health.