I added some black butter cream scroll work to white buttercream and the black color bled a little onto the white after is set for a while. Is there a way to prevent that?
Did you make the black just before you used it?
Was the white wet (condensation)?
These are usually the reason black w/bleed.
Also use less paste color in the balck by starting w/chocolate or dk blue or mix lots of different colors (tiny bits of leftovers) together before using the blk color.