MEXICO CITY — Like a character from a graphic novel, she dresses in black, has unusually blond hair — and kills bus drivers who sexually assault women. In a place like Ciudad Juarez, known for its years of brutal killings of women, the story has inexorable appeal. But how much of it is true? Authorities are taking the reports seriously enough to investigate and have posted undercover cops on buses. Women's advocates say they wouldn't be surprised if someone finally had taken long-denied justice into her own hands.