I have a bride who wants white buttercream on her 3 tier cake. She wants the whole thing piped w/ scrolls and swirls in black. What do you suggest I use to do the piping? And how long does it usually take for the black to start bleeding on the BC? Is it preventable? Thanks for any help!

Use chocolate frosting and color it black. This way you will use less coloring and thereby have less worry about 'bleeding'.

Besides, in my experience, bleeding only happened if the black icing was super-saturated with color; sat for DAYS; or was on a decorated cake that had been frozen and thawed, and the resulting condensation caused the bleeding.