Pigmentation of the gums (dark color seen at the gingiva) is generally physiological (normal) depending on the person's race. African Americans present more pigmentation, thus gums may have a darker color, especially in some areas. This is generally seen symmetrically (present both in right and left sites).

Sometimes, darker color may happen due to a foreign body. It generally occurs due to amalgam restoration materials staying around following tooth extraction, etc. [It looks like a tattoo.]

And sometimes, they may be related to a local pathology affecting melanin-releasing cells [very similar to what we experience on skin].

See a dentist.