I start every year by teaching my students how to use chalk pastels. I have found that students get really upset when they have worked so hard on their project to have it "ruined" by not using the pastels correctly. So, I demonstrate some of the things that have made students unhappy with their art work in the past.
Using the chalk like crayons coloring back and forth - it makes a mess and then there isn't room to blend the colors.
Pushing too hard with the chalk - it makes lines that no amount of blending will get rid of.
Wet chalk (sometimes it gets that look from just the students sweaty hands) - it makes the original lines darker, but it does not blend well.
Blending - with most of the projects that I do I want the color to gradually move from one color to the next, not three distinct color lines.
Then I demonstrate on the leaf and sometimes on another apple how to blend the colors into each other. I go around the black lines with the dark green and then put some light green next to it. Then I blend the colors together so that you can see the original colors, but other greens are created as well.