Hello sir...
If the pigmentation (Black marks) is present since birth ,it is NORMALone.....
If it is acqired in the recent days, It may be due to
**Poor oral hygiene
**smoking and alcohol
**Anemia
**certain medication
**Fungal infections
**Dry mouth
**Hyperpigmentation which occurs by itself or a genetic condition
However there is some chances of you suffering from MELANOMA, which is a cancer type...Melanoma occurs if UV rays penetrates in to mouth .....
Next Steps
kindly visit nearby dentist to rule out the cause nd to treat the problem
Health Tips
If have any habits of alcohol and smoking ,quit that immediately