The frequent lover of Spider-Man is also periodically one of his foes. Black Cat is a burglar, who takes whatever she wants and desires. It makes sense for her to dress in all black, as this will help her avoid any detection during night.
What eludes me is how she has managed to keep her identity secret for such a long time. Her face is barely covered by a black mask. Are people daft and unable to recognize her as the famous Felicia Hardy?