A few months ago i developed a fever blister. now when i tan i have white skin all around my mouth and a lot of white skin where my fever blister was.

Sunscreen: You likely have some hypo pigmentation from the inflammation. If you have scarring in the area it's important to protect it from the sun for at least 6 months. Sun exposure on the scar tissue can cause it to have a different color permanently. Use a sunscreen with SPF of at least 30.

May improve: its not unusual to have some hypo or hyper pigmentation after a cold sore heals. Usually this will go away, but you can encourage that with vitamin E oil or borage oil or aloe on the scar. Also use sunscreen.