Sooty creates a dark or black covering on the back of your dragon. It should have interior holes that get more numerous as they reach the edge of the mark. 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.
You may have the marking cover up to 75% of the body, as long as it appears on at least half of the back.
Charged Sooty may be lighter or white, rather than darker.