I haven't had this happen in a few years, but I remember my buttercream was too wet when I piped on the black icing. Putting the icing in the fridge and then letting it sweat when its pulled out of the fridge will let it bleed. Make sure everything is dry and firm. I hope this helps. Maybe someone else has a better idea that I didn't think of.
Good luck!!!
I juat had this same exact problem with a FBCT yesterday! I have never had a problem with my own black BC bleeding but I used the Wilton Black from the tube and it bled like crazy! I am thinking that if you made your own BC black, that you made the consistency too thin (too much water), causing the color to bleed.
I had this happen a lot. then I realized that if i put my cake in the kitchen under the ceiling fan for a couple of hours after it was completed, it would crust just enough to stop it. I don't put them in anything other than a paper box to store and transport after that. I used to put the ones that would fit in a rubbermaid cake caddy and the moisture made it bleed terribly.
This happened to me with a FBCT. A little blotting took care of it but it was still stressfull!
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