I would say let your cakes BC dry real good once its iced, then get a good stencil of the design you want....what I did with a stenciling I did once was, let my bc dry, then got my stencil, and I also used bc, but alittle more softer, almost to the texture of runny pudding, and I placed the stencil over the cake and used a plastic card to stencil my design.
Be sure to lift your stencil off straight up so it doesnt streak.
It came out good...good luck!
I just recently used black royal icing to stencil on a white buttercream cake (I have posted photos here). My advice is to let your buttercream crust over really well. I was also able to refrigerate the smaller tiers before stenciling and that worked well also. The largest tier was too big for my fridge, so I just let it crust over before attempting the stenciling. Just take your time (and a deep breath) - it does work! Lift the stencil without moving it sideways or up/down. I practiced a few weeks in advance with my buttercream on styrofoam dummies to make sure I had the technique down and that the black did not bleed. I suggest taking the time to practice - it will increase your confidence when you actually stencil on the actual cake! Good luck with it 
)
I once saw a website where this guy did nothing but damask on butter cream. If it is anything like where when I worked in a local cake shop, she iced on frozen cake and then put it in the freezer to set up. The combination of frozen cake and cold buttercream would have made it easy to stencil on.
Hopefully the cake thawed before any of the events.
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