How to remove dark scar due to chemical reaction (salicylic + phenol) ?

A problem +/-success: Scars are injury reactions. Using toxic chemical agents, as you mention, induces additional injury. Thus outcome could easily end up worse. Strictly speaking, careful excision ; closure using plastic surgery techniques is best option ; I have personally done lesions with excellent results. However, surgeon is not in control of healing the wound; the body owner is ; great work can easily go bad.

PIH: Post inflammatory hyperpigmentation is not easy to treat and injuring the site further will likely cause further inflammation and possibly further reaction. See a plastic surgeon.