Darker fashion abounds in SL. Just join some of the Goth groups that are around.
The Thrifty Goth, The Eldergoth Macabre, Dead Dolls of SL. (Sorry not in SL atm to give links)
My avatar is a Demon and I adore darker fashions, but I stay away from the stereo type vampire stuff. There are LOADS of shops ranging from cybergoth to dollygoth, to classicgoth, punkgoth, and the list goes on. Each of the groups I listed have lists of their sponser stores that would be a great place to try.