According to How Stuff Works, black gums are the result of black gum disease, otherwise known as acute necrotizing periodontal disease. An imbalance in melanin production or pigmentation in the gums is another likely cause.
How Stuff Works further notes that black gum disease degrades the gum into dead tissue. Other symptoms include an offensive mouth odor, bleeding and pain. Malnutrition, smoking and HIV can lead to black gum disease. It can also spread to the bones and cartilage around the gums if the problem is not treated. Treatment for this illness comes in the form of oxygen therapy in conjunction with antibiotics. Removing the decaying gums through surgical procedure is another option.
Wikipedia claims that the illness occurs in various stages, including the milder necrotizing ulcerative gingivitis. How Stuff Works claims that acute necrotizing ulcerative gingivitis is also known as "trench mouth" due to the onset of the disease among soldiers during World War I. This form of gingivitis is the result of poor dental hygiene, but it is considered a rare disease in modern society.
How Stuff Works adds that a melanin imbalance in the gums is more likely among people with dark skin; however, such an offset in the gums can happen in any ethnic group. Other diseases in the form of Peutz-Jeghers syndrome, Addison's disease and Von Recklinghausen's disease can cause a boost in melanin production. The gums can also darken when exposed to too much sunlight. Certain cosmetic procedures lighten and restore proper pigment to the gums.