Well, darker in general, I would recommend Jigoku Shoujo (Hell Girl). It is about people seeking vengeance and a website that provides it for them. Hellsing is pretty amazing, and a vampire one. Vampire Knight is a bit lighter than Hellsing, about a school with a day class of regular students, and a night class with vampires. Confidential Confessions is a darker one, but with no fantasy element. It is different stories of people going through hard times. Gravitation would be along the lines of funny and slashy. Fruits Basket would be a good mix of funny, dark, supernatural. I hope this helps some, I will pass on the request to friends to see what they come up with too.