Does anyone know how to stop black buttercream from leaching into the other icing? I do black outlines on a lot of my cakes and sometimes the black runs a bit. (Usually if the weather is hot or the cake has been sitting a while)

you can try piping the lines first with white (or whatever your background color is) then over pipe with the black, it shouldn't run as far cause the edges of the lines won't be touching the cake.