OK from experience I know that black icing bleds really bad on a white background. I have a request for a cream cheese icing with alot & I mean alot of scroll work but they want the scroll work in black. Should I tell them to choose another color? any suggetions thanks in advance 
First think I might do is make your black icing from dark chocolate icing so you dont have to use so much black. Also you could use some meringue powder to help stablize it. I have a cake with a lot of black and white (hibiscus cake in my photos) and i didnt have a problem with it bleeding into the white. I didnt use meringue powder either, I just got the chocolate icing really dark with the dutch processed cocoa first.
Or you could see if they might want brown and use chocolate. I think that would be pretty too...
I just made black (it was 1/2 crisco 1/2 butter BC) and used it on skin colored icing that didn't bleed at all - see my photos, pirate cupcakes. I used Americolor Super Black to tint white icing and it was fine.
Previously (for cookie monster eyes) I used an off brand black gel and that bled. I think the problem was that I used water to thin it out AFTER I had added the black, plus I highly recommend the Americolor Super Black. Good luck!
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