Blow Pens are water colour markers, so they will wash out pretty much right away if you bathe the dog, or if it rains. Some of my friends with smooth coated dogs (or #7'd dogs) just draw right on their dogs with crayola markers.
Blow Pens is a brand. It's a water colour marker, and you blow into it, and it creates an airbrush effect. You would need to use cards/stencils to block some of the brush if you are trying to shape something.
They are available in the kids/craft section of major toy stores, or large crafts stores (Michaels.)