Sooty creates a dark covering on the back of your dragon. It should have interior holes that get more numerous as they reach the edge. These may be soft or hard. They can be smooth or they can resemble the Sooty texture of real-world horses. Here is a brush set if you can’t quite pin down that textured edge: Dapple brush .
Here’s a video that shows different methods of making the sooty edge:
Charged Sooty may also affect the outside of the dragon’s legs.