I don't have my books with me at the moment - what a shame - but I can give you some thoughts on it. As you well know no one ever ever seems to talk about her dress online. Most seem to center on her physical appearance. Perhaps this is due to the fact that she doesn't seem to fond of wearing clothing. Maybe that's just my thoughts on it. For some reason I always think of her in red. But alas we rarely see fan art of her. The boys are just too pretty. Not that I'd complain.
The Anita Blake Compedium describes her as such: Belle Morte is pale skinned with long dark hair. Her eyes are pale brown, but shine with a dark honey light when she is using her powers.
The Page of Eternal Life stays much along the same lines: Belle-Morte is a pale, dark-haired woman. Her face is like something carved out of alabaster, with lips red and perfect, hair like the darkness of night made into furred silk, falling around her perfection. Her eyes are pale brown, like dark honey.
Purple Passion has some decent fan art. Not the best I've seen but the only pieces of Belle Morte I've ever seen.
Charmeye's Idea of what Belle Morte's Gown would look like. But as I said.. I imagine red where she puts white.
Hopefully that provides some help. If I can pull out my books and get accurate ideas on her dress, I'll let you know. And since you are such a good fan I'll share a new link I found with ya. Schurry's Anita Blake Site. It seems to be a good one.
All the best