I am making a wedding cake tomorrow. It will be white buttercream with black scroll-type piping and roses. I had one experience where my black icing bled into the white a few hours after I made it. I don't know what made that happen. Anyway, just want to make sure that doesn't happen to this cake.

Yes, I would let the white icing crust first. Also, if you use choclate for the base of your black icing, then it doesn't require so much food coloring and is less likely to bleed.