It's never happened to me before...now i made a sponge bob cake that i'm being paid for..i made it a day ahead cuz i had quite a few cakes for this weekend and now i look at it and the black has blead onto the yellow and white...i guess i'll just go over it before i bring it to her..but this has never happened to me before and now all of a sudden my black is bleeding??
Is this normal?? does it happen to anyone else?
It's been in the fridge the whole time...i dont understand 
I've done lots of cakes using black buttercream for outlining, and I have found that the cake really needs to be done on the day of the event in order for bleeding to be minimalized.
Why is your cake in the fridge? So many people do that here and I don't understand it. Condensation can wreak havoc on cakes, particularly ones with lots of colors - especially dark colors.
Your cake turned out cute ~ SpongeBob rules!! 
I had never had this happen to me either until I did my Darth Vader cake. The customers are lactose intolerant and asked that I not use milk in my frosting so I used water instead. For the first time, I experienced the bleeding colors. It has to be the water or condensation if you had it in the fridge.
I use the all crisco wilton buttercream recipe.
Hope this helps!
worst colour bleeding i ever had was early in my career when i did red buttercream script on a white whipped icing cake and froze it overnight (they were picking it up in the morning). holy pink aura around the writing! from that point on, if i were writing on a whipped icing cake, i would first do the script in a large tip in white buttercream, and then went over it in the coloured buttercream.
moisture definetly "pulls" colour from any decoration (darker buttercream, marascino cherries, etc.) fridge condensation wreaks havoc on colours, and also on cooked sugar (melting it).
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