I can understand your concern.
Lichen planus pigmentosus
is characterised by dark brown to slate gray pigmented skin lesions and unfortunately these do not resolve completely as pigmentation is deep.
The treatment modalities supposedly giving desirable results are
Use of sunscreen with SPF 50
Oral vitamin c and antioxidant supplement intake daily
Application of steroid cream or tacrolimus ointment along with hydroquinone
4 percent cream
If the lesions are spreading, oral steroids might be required
Oral vitamin A
analogue like acitretin
Some advocate chemical peels
or lasers to improve the lesions cosmetically.
Hope that helps. Take care