Thanks for your query, I have gone through your query.
The black color of the mucosa or inner aspect of the cheek could be a normal physiological pigmentation or increased pigmentation because of chronic irritation because of smoking.
The thicker mucosa could be because of
frictional keratosis or leukoplakia or liche planus or
candidiasis.
Consult a oral physician get the above said conditions ruled out.
if it is frictional keratosis, remove the cause like sharp teeth by rounding off the sharp edges.
If it is candidiasis then you have to take
topical antifungal like candid mouth paint.
If it is leukoplakia, you have to stop the habit of smoking and take antioxidants like capsule lycopene.
if it is
lichen planus then you have to take topical steroids like
triamcinolone acetonide (0.1%) 4-5times daily for 1 week.
I hope my answer will help you, take care.