I had the same thing happen when I made a royal icing transfer last year. Here's a link to the mess:http://letsgetcaking.blogspot.com/search/label/Monk
I've discovered that even if the black outlining is completely dry, you will still have bleeding problems if your filler icing is too wet. You do need a flow consistency of icing, but make it as thick as possible and still get a smooth surface, and you'll minimize bleeding colors.
I've had the best luck with using plastic wrap. I've tried waxed paper and the pieces seem to break easier. If you use plastic wrap, just flip the icing over once it's dry, and peel the wrap off the back. You just need to make sure to tape the plastic wrap in place to keep it from moving and make sure you have no wrinkles in the plastic before you start piping.
I hope that helps some.