If there is an internal problem with your engine, there will be "sparkles" (metal particles) in the residue. If not, the residue is nothing more than combustion products. The residue will be more blackened if your fuel contains castor oil, than if it has only synthetic oil. It will also be more blackened by air leaks into the exhaust system -- this permits greater oxidation of the hot oil/fuel vapour in the muffler.

If no sparkles -- no problem.

Multi-blade props (three or more blades) do not perform well on models -- they are used to increase ground clearance &/or to provide a more "scale" appearance. You did the right thing by switching to a 2-blade prop.

Enjoy your new hobby.