Blackhead

Comedones, also called blackheads, are the primary efflorescence of acne. Blackheads because they thought they saw originally living parasites.

Comedo occurs when the follicular canal of the sebaceous follicle becomes blocked due to excessive keratinization. Horn lamellae condense into a plug which completely closes the exit of the follicular canal. The melanin stored in combination with the oxygen of the air can cause the plug to turn dark.

https://de.wikipedia.org/wiki/Komedo _{(Wikipedia CC-by-sa-3.0)}