One month is a significantly long period of time and warrants consultation with either a dermatologist or oral medicine specialist.
Blackening of an ulcer can occur due to blood clots, infection and a range of other cases and it is difficult to comment without a picture and more detailed history.
Next Steps
1. Immediately consult a doctor
2. Apply glycerine on it for temporary relief
Health Tips
Your health is in your hands, neglect is an unfriendly and unforgiving path.