是一個短語，就是指好看。dark可以指頭發，也可以指膚色，或者mysterious.

貼一下查到的一個例孑做參考

For example, in Jane Austen’s classic Pride and Prejudice (1813), she implies the leading man, Mr. Darcy, was tall, dark and handsome, though she doesn’t exactly use that description. The only description given of Mr. Darcy features are “…he soon drew the attention of the room by his fine, tall person, handsome features and noble mien.”

However, if dark is referring to mysterious, then Mr. Darcy would fit the description and in most adaptations, he is shown to have dark hair. Considering Pride and Prejudice is a foundational novel that many romances, both movies and books, take some direction from, it is no wonder that a majority of leading men have been cast in this description for over a century.