Why Doesn't The Black Marks Heal Even After Years?

my skin always becomes black on healing any wounds..it wont go even after years..i got a small burn on hand when i was 7 .it remains as such and now im 18!this is the case with all my wounds and even pimples..i had not got much pimples during my teens,but the problem is that all pimples from the time it had started leaves black spots on my face,with no intention of vanishing at all..
height 5'6
skin :normal