Have developed a new blue and black spot on lower jaw in old extraction spot I have bad gum disease, only have 17 teeth left, very bad tmj. Cancer?

See Your Dentist. Please see your dentist to have this new development evaluated and determine if your gum disease is being controlled. .
Watch carefully. While a pigmented area or growth on the mandible is most likely benign one should evaluate to assure lesion does not progress in size. Most pigmented bucal lesions are benign and related to various hemangiomatous processes. Malignant melanoma of mucosa while rare is seen and occurs in anal can, esophagus, vagina and oral cavity. If changes noted get bx.
Black blue spot. Any unusual changes in your mouth or on your jaw, whether painful or not, that lasts longer than 7-10 days, needs to be examined. A diagnosis can only be made after a clinical and visual examination. Make an appointment and see your dentist, a Periodontist or an oral surgeon.