Hello and thank you for your question.
Based on your question it could be a few things:
-Amalgam tattooing from old fillings
-oral melanotic macules
-heavy-metal pigmentation (lead, bismuth, arsenic, mercury)
These are the main ones I can think of. Google photos of these lesions to see if they look similar. To be honest, I can't give you a proper diagnosis which out evalution and possible biopsy
Hope this helped.