There are several possible causes for darkening of the gums near the tooth line.
First, it is possible that this is a normal finding for you. As people age, the color of their gums sometimes changes, particularly in certain ethnicities. This is not usually a problem nor a sign of an underlying medical condition as long as the changes in color are uniform and not associated by any other symptoms.
If on the other hand, the gumline is puffy or inflamed, or if brushing or flossing causes a significant amount of bleeding
, then it is more likely that the color changes are related to a condition called gingivitis, which is an infection of the gum tissue. In addition to stepping up your oral hygiene (more frequent brushing, flossing, and mouthwash), you would need to see your dentist as soon as possible, as gingivitis can become quite serious if untreated.
Finally, it is possible that the changes in color that you are seeing are more related to tooth discoloration, which is in turn making the adjacent gums appear darker. Tooth darkening is worsened by smoking, but also by tea and coffee intake. A good routine cleaning at your dentist's office may help.