I havent had any problems with black bleeding before. Here is what I do:
Start with chocolate icing, then add the black.
You could also try adding some meringue powder (I dont, but that is what it is for)
I have had that problem, too. I hope this may help you. I found it I stiffened the black icing with confectioner's sugar it prevented it from bleeding. I used a lot of sugar so that it takes quite a bit more pressure to squeeze the decorator bag. In my photos is a cake, "black scrollwork" that I used this method to stablize the black. The cake sat for 24 hours before I took the picture. I use this method to do FBCT as well.
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