Hi Dear,
Welcome to Health Care Magic Team.
Understanding your concern.
The condition you describe in your query black discoloration is clear indication of
allergic reaction ,which can occur due to some factors like over exposure to sun ,irritation of skin due to some dental product (like fluoride toothpaste).excessive saliva in mouth and due to deficiency of certain vitamins specially
Vitamin B12 . It could be because of hyper pigmentation in that area when ever increased exposure to UV rays of sun is there. UV rays causes absorption of melanin pigment and dark patches form.
So its very important to find the causative factor , so that it can be treated accordingly .
I would suggest you to apply creams like
hydroquinone and azelaic acid . you can also take oral anti-inflammatory medicine like
diclofenac ,also take some vitamine B12 supplements .Do not lick your lips .Apply lip balm to your lips with clean hands before going to bed and drink lots of water .
If condition still persists then
consult dermatologist .
Hope your concern has been resolved.
Best Wishes,
Dr. Harry Maheshwari