According to WebMD, blackheads occur when sebaceous glands become blocked and oil builds up under the blockage. The oil sediment turns black because it oxidizes when it is exposed to the air.
Hair follicles and sebaceous glands occur in close proximity to one another in the skin where they form structures known as pilosebaceous units. According to Wikipedia, pilosebaceous units are most commonly found on the face, neck, upper chest, shoulders and back. Accordingly, blackheads and other forms of acne occur on those areas of the body.
The underlying causes of blackheads and other types of acne vary from individual to individual. WebMD notes that most people suffer from acne to some degree over the course of their lives. Hormones can influence the occurrence and severity of acne; many women break out at specific points during the menstrual cycle. Testosterone levels have also been associated with the formation of acne. Some foods have also been shown to trigger acne in certain circumstances. These include high glycemic index foods and dairy products.
According to Simone Kitchens for the Huffington Post, there are several ways to treat existing blackheads and prevent the formation of new blackheads. Exfoliating the skin with a mixture of baking soda and water can help minimize the appearance of blackheads. An apple cider vinegar solution applied to the skin can help clean out pores, reducing blockages in the first place. Clay masks have the potential to absorb excess oil, reducing the risk of blackhead formation.