skin. A blackhead is a type of acne vulgaris. Contrary to the common belief that it is caused by poor hygiene, blackheads are had by excess oils that have accumulated in the sebaceous gland's duct. The substance found in these bumps mostly consists of keratin and modified sebum (an oily secretion of the sebaceous gland), which darkens as it oxidizes. Clogged hair follicles, where blackheads often occur, reflect light irregularly to produce a blackhead's "black" hue. For this reason, the blockage might not necessarily look black when expressed from the pore, but it can have a more yellow-brown color as a result of its melanin content.
In contrast, a "whitehead" (more commonly known as a pimple or a closed comedo) is a follicle that is filled w/ the same material, sebum, but lacks a small opening to the skin surface. Since the air cannot make the follicle, the material is not oxidized, and remains white.