I got an order to make a black and white wedding cake next month. I want to do white cake and then make a bottom border ribbon out of black fondant, then decorate the black fondant with white buttercream piping. But I just can't seem to figure out how to cover the seam of the ribbon without adding a bow or flower over it, which I don't really want to do. Any suggestions?

That's why every cake has a "back"!

You could try making a paste from the black fondant and a little water and spackling the seam. It still kind of shows, IMO. But with piping BC over it, it won't be noticeable. (and put that part in the back!)