I know you've heard my views on wearing white to a wedding...any amount of white. However, sometimes (emphasis on the sometimes) if you pair just the right amount of white with black, it can be an appropriate, formal wedding look. You do have to be careful though.
First, you want to make sure that the dominant color is black (i.e. a black dress with white accents). Second, make sure the silhouette is not too "bridal." Don't get a long mermaid-style dress!