Someone's gums may be black because of genetics, ethnicity, disease or medication, according to HowStuffWorks. Higher clusters of melanin or uneven pigmentation can result in dark gums, particularly for dark-skinned races.
Black gum disease, or acute necrotizing periodontal disease, causes black gums because the gum tissue is dead, explains HowStuffWorks. There may also be bleeding, pain and odors. Smoking and HIV are some causes of black gum disease. Antibiotics and surgery can treat the condition.
Acute necrotizing ulcerative gingivitis causes a gray film that may make gums appear black, as stated by HowStuffWorks. It was common among soldiers during World War I because they spent a lot of time in the trenches and were unable to care for their teeth.