Which answer most accurately describes a blackhead?

A blackhead is most accurately described as an open comedone. Comedones are formed when hair follicles become clogged with sebum (oil), dead skin cells, and debris. In the case of an open comedone, the top of the clogged pore remains open, which allows exposure to air. This exposure results in the oxidation of the mixture within the follicle, giving blackheads their distinctive dark coloration.

In contrast, closed comedones, commonly referred to as whiteheads, form when the clogged pore is sealed off with skin, preventing exposure to air and thus not undergoing oxidation. Pore congestion may describe the overall condition of multiple clogged pores, but it does not specifically define a blackhead. Similarly, while blemish is a general term for any skin imperfection, it is too broad to specifically identify a blackhead. Thus, "open comedone" precisely characterizes the nature of a blackhead in both its formation and appearance.