The BK knife blades are usually sharp when you get them....only need to strop. 400 grit is pretty rough and would suggest you go to a finer paper to get a finer edge...........sounds like you might have actually made the edge less sharp and polished....hope this helps........MIke
If I recall correctly, he is drawing with a stylus and then using a modeling tool to "erase" with. It would also explain dark "smudges" from the light burnish effect from the modelling tool.
Mike