Purchase liquid chalk markers from a craft supply store. These markers have the smooth texture of paint, unlike chalk, which can be dusty and rough.
Use the markers on the blackboard to write a message or create a scene. Do so carefully, since you can't easily erase the liquid chalk as you go.
Dampen a washcloth and squeeze out any excess water so that it's not dripping. Use that to wipe off the liquid chalk when you're done creating a painting or ready to start all over again.
Clean the blackboard surface by using an appropriate spray cleaning product. You should not only wipe up any chalk and dust, but also make sure that fingerprints, oils and residue are wiped away as well.
Sand down the chalkboard surface using fine, 220-grit sandpaper. This will prepare the chalkboard for the paint that will go over it.
Apply a thin coating of an oil-based primer to the clean, sanded chalkboard. Let it dry completely before you check to make sure that there are no areas where the chalkboard surface is not covered in primer. If necessary, apply a second layer of the primer and allow it to dry as well.
Paint over the primer with the latex-based paint of your choice. If you'll be combining colors, remember to do so one layer at a time. Each coat should dry fully before you apply the next.