Anyway, I decided to try a few different application methods to see if I could get the icing to bleed on purpose. I used four different methods, some cold and some room temperature, to see which ones would or wouldn't bleed. I used crusting buttercream for all of them, half butter half shortening.
The first one was white icing that had been refrigerated until it was hard, then I stencilled the black design onto it. It was then returned uncovered to the fridge and kept cold. There was no bleeding at all that I could see.
The second one was crusted over at room temp, piped on with the black, then left uncovered at room temp. Other than the beautiful subject matter, there was nothing notable about that one either. No bleeding of the black color at all.
The next one was crusted over, piped on with the black, then put uncovered in the fridge. No bleeding here either. I even stuck it outside uncovered for a while, and I think that it was molested by something (I suspect Lloyd) but it didn't bleed. It was a cool day, though, so if it was hotter I think there might have been some color running.
Now this one...this is the only one that had any tiny amount of bleeding around the edges of the piping. It was stored in an airtight, sealed container. When you seal things up in an airtight container they tend to soften up, and I think that the icing in this particular arrangement stayed softer and was able to reabsorb the moisture from the icing that was circulating around in the container.
I don't know that you can see the bleeding, it's very minimal. I suspect that if there had been more cake and more icing in the container there would have been more moisture, and more bleeding.
So I think the secret is to let the air circulate freely around the piped icing, whether that's in the fridge or out. Don't close up dark colored icing in a sealed box, because that will allow the moisture to recirculate and soften everything up, and will result in more of a chance of the color bleeding out.
It also helps if you use the least amount of food coloring that you can to make the black color. I start with white icing, add cocoa powder to that, then add airbrush color to that. The cocoa powder dries the icing out, then the liquid color softens it up again, let it sit overnight because it will continue to darken if you leave it for a while.
Kara Buntin owns A Cake To Remember LLC in Richmond VA, and cake supplies online at www.acaketoremember.com and www.acaketoremember.etsy.com