Some of it is layering the marker over the dried tone and other parts are blended 'wet-on-wet'. I didn't use a blender marker. I've never really achieved good results using the blender.
For some of the really dark areas, I had to put on up to 8 layers of the same marker color.
For some parts, I also used pencil crayons to put in hi-lites or dark shadows and a bit of texture.