A signing that could be good news to the millions of Americans, especially teenagers, who suffer from unsightly acne was made for Atria by editorial director Tracy Behar. She won a heated auction for Unblemished by Drs. Katie Rodan and Kathy Fields, star dermatologists who run the Proactiv line of skin care products well-known from their TV ads; they offer in their book a three-step formula they promise will get rid of the scars forever. It involves only products that can be bought over the counter in drugstores for self-treatment. Behar bought world rights from Barbara Lowenstein at Lowenstein-Morel Associates and will publish next spring.