Is there any kind of "magical" way to keep black buttercream from bleeding/hazing into white?
Sorry i don't know,but i'll love to.Are you doing like a two tone design.
Waht about building a little dam for the color to stay put, or have you tried color gel? i dont know maybe if you post a pic we could be of more help.
It helps if you wait until your white buttercream has crusted (if you're using a crusting recipe), before you pipe any black icing on it.
I used the pre-made, pre-tinted tubes of Wilton icing that were black to do a cake recently and I was amazed that it didn't bleed. Or maybe I just got lucky lol