You will see more carbon (soot) from the kerosene than you will from the castor oil. If your kerosene is somewhat "dirty" (as in less refined than say Jet A), you will see more soot.

As for blending of oils - it is my opinion that any engine using a ferrous piston and liner should have only castor oil for lubrication. While synthetic oils tend to keep things "cleaner" internally, they do tend to prevent the castor varnish from forming on the piston which is important for sealing the piston within the liner. Better piston sealing = better power.