Lois: How does a guy with 9 phones not return a phone call?
Clark: Oliver's probably celebrating with someone else.
Lois: Wow. Tall, dark, and single. (I've always thought that dark stands for
someone with black hair but Oliver is blond? Is my assumption correct?) Go figure.
"Tall, dark, and handsome" is a collocation - it's a cliche used to describe a desirable man. She's making a play on that. It's been in use so long I don't think anybody even thinks about the literal meaning of the "dark" part, but I suppose it could be taken to mean "mysterious."
Thanks, D!