There are many reasons for darkening of the gums. If you believe the area is spreading, it is important to have it evaluated by your dentist.
This discoloration is known as pigmentation of the gum. It is a normal condition for many people. Dark discoloration is known as melanotic pigmentation. African-Americans tend to have more melanotic pigmentation of the gums.
Pigmentation does not, however, grow in size over time. When your dentist examines you, he or she will compare the current situation with notes on the discoloration that would have been made at your previous visits.
Smoking can contribute to darkening of the gums. This is known as "smoker's melanosis." This condition increases with age and is present more often in women. There is no treatment, but quitting smoking will decrease the darkening.
Another reason for dark discoloration of the gums is an "amalgam tattoo." Amalgam is a mix of metals often used in dental fillings. The "tattoos" appear in areas of the gum that are in close contact with amalgam material.
I would not expect to see a new amalgam tattoo in the gum around a tooth with a crown, as no amalgam is in contact with the gum. However, you could have a tattoo from a filling that was present before the crown was made. You may not have noticed it before. During an exam, your dentist will compare the new area that you are describing against any previous notes about the area.
Finally, a dark mark also can be a sign of melanoma, a type of oral cancer. If your dentist is not sure what the dark area is, you should have a biopsy. Your dentist can do this or refer you to an oral pathologist or oral surgeon. It is important that you have the area you are describing evaluated.