A blackhea (medically known as an open comedo , plural comedones ) is a yellowish or blackish bump or plug on the skin. A blackhead is a type of acne vulgaris. It is caused 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 (resembling dirt) as it oxidizes.