Black soot is unburned fuel. I have yet to see a diesel spew out black soot on B100 but at over $4 per gallon for B100 or D2 why would anybody want to waste fuel at these prices?
Once B100 has cleaned out your fuel system and your fuel filter has been changed to remove the crud, switching from B100 to B50 to D2 and back is fine. During winter B100 will tend to gel so blending with D2 is what most do.