Often, a black long hair girl in cartoons might be depicted as mysterious and alluring. Her hair could add to an air of elegance and maybe even hint at a deeper, hidden side to her personality.
Well, often in such cartoons, the girl could have a confident and charming demeanor. The natural hair might be shown with lots of texture and volume, and she might be dressed in colorful and stylish outfits to reflect her personality.
Another horror story might be about bad haircuts. A natural black girl went to a salon where the stylist didn't understand the unique texture of black hair. Instead of giving her a nice trim, the stylist cut her hair unevenly. Some parts were much shorter than others, and it took months for her hair to grow back to a somewhat presentable state. It was really embarrassing for her at school and social events.
The attractiveness could come from her unique style, the way she expresses emotions, and the way the artist portrays her with long, flowing black hair. Also, maybe her personality and the story she's part of contribute to her appeal.