When my students use oil pastels I tell them to use black last. They used
to outline with black first and then the colors would smear. Now they use
it as a final touch (if they want to) and it works great. Also, by using
black last, it prevents them from coloring large areas black which makes a
mess on their hands and papers. I wish I had thought of this simple rule
sooner. Even the kids appreciate it. Every now and then one of them
forgets and says, "Oh, I forgot to use black last!"
Sky in NJ