It means that an ingredient or product causes the formation of comedones (blackheads) in a relatively short period of time. Blackheads form when the outer layers of our skin do not shed properly and the hair follicle is blocked. The blackhead part comes from the oxidation of fatty acids on the surface in the skin. Scienticians still aren’t really sure what causes this lack of desquamation. Formation of comedones are not accompanied by skin redness.