A blackhead (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. Blackheads are typically caused by excessive oil and makeup, which can facilitate the multiplication of the bacterium Propionibacterium acnes, the predominant anaerobe of the normal skin flora. 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. 
For information on the causes and treatment of blackheads, see the above linked acne vulgaris.