I just helped a friend accessorize her GMOB dress for an autumn wedding, and the tricky part was she bought a sleeveless gown she loved and then was unable to find a topper that pleased her. Eventually she bought a second dress that came with its own topper.
I would beware sleeveless in a Canadian autumn unless you already have a fab topper in mind, or know you won't need one for whatever reason.
FWIW, I think it's no longer considered bad luck or overly somber to wear black to a wedding, especially if the dress has a little glitz. If you google "black and white weddings" you can see plenty of images where the whole bridal party is in black, and sometimes even the cake is a BW theme.