It can be due to hyperpigmentation lips. To prevent blackness of the lips, smoking and chewing, biting or pricking the skin of the lips should be avoided. The most effective solution for treating dark lips is alexandrite and ND-Yag Q-Switched laser treatment. Please discuss these options with your dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.