hi..Thanks for the query.
A lot of people have dark lips genetically predetermined,although parents may/may not have it. A frequent cause of non genetic lip darkening is the use of agents that might irritate / bring about a contact dermatitis
in the lips.
I would advise you to start using medicated lip balms with SPF and to reapply it several times in a day..that helps in reducing further darkening.Also use a cream containing kojic acid
twice a day for lightening of lip colour.U can also opt for mild chemical peels
once in 15-30 days to exfoliate better and lighten faster.
Hope this is helpful.Take care!