Gum color varies. All gingiva (gums) are "not created equal" - some are pink, some are grayish color, and sometimes areas are almost black. This can be due to pigmentation, the same way skin color is not all the same, sometimes even on the same person. You didn't mention if this blackness is new, but make your dentist sees this - some diseases can produce color changes, and they can be serious sometimes.
Melanin. There are a number of things that can cause tissues in the mouth to be dark. One possibility is the increased production of melanin that is responsible for the dark pigmentation in tissues. In some countries and races, black or almost black areas may be noted in the tissues of the mouth, especially the gum or gingiva.
Genetics. Often it is a result of genetic background depending on amount of pigmentation.It often can be treated by a dentist employing a dental laser.
Serious or Minor. Spots on the gums are usually due to a increase in pigment and is more common in dark skinned individuals. However, some spots on the gums can be very problematic (some cancers present with dark spots in the gums). If you have yet to get them reviewed by a doctor, please do so. Depending on your race, gender, other risk factors, a dentist or specialist could lead you in a better direction.