oh, I didn't notice that the skirt was black either way. Well, as chenoameg said, I don't think you should wear black at weddings (in_parentheses, I know you and I disagree on this, but I think weddings are all about symbolism and symbolism of black and our culture is historically inappropriate at weddings, to me). And I think a black skirt is too much black. But plenty of other people will be wearing black, especially since it's a formal wedding and plenty of people just where their LBDs to formal weddings, so the real reason not to do it is if you're aware of the symbolism and it would make you uncomfortable. I would err on the side of more casual and not black, myself.

Miss Manners points out that the problem with an invitation like "formal but not black-tie" is that nobody knows what it means, so some people will be wearing cocktail dresses, and some people will be wearing formal dresses, and some people will just be wearing church skirts and tops. So I don't think you need to worry too much about the formality.