I am piping black scrolls on white icing but my black buttercream icing bled on the white I have stripped the cake down and started overbut how can I avoid the black piping bleeding again?

It's my understanding the newer (freshly made) the blk icing the more this will happen. Make blk way in advance - and if you made it by mixing together *all and every* color you have (depending on how much you need you could take just a tablespoon of each color to mix together), then add your blk color it won't take near as much to come up with blk. Another way is to make a dark chocolate icing to add blk color to.

In all my yrs I can't think of a time I had blk bleed into my white. Is the white dry? Crusting b'cream? Yes, I've heard of it but don't remember it ever happening to me.

Perhaps black royal icing?