Coating non-rusted surfaces with Rust Reconverter will not really stop erosion and rusting and will not result in a black coating. That is not really a bad thing; it will be like painting the surface with a clear latex paint.  It may not bond as well as it would to a rusted surface and it won’t really serve as a corrosion preventative barrier but it can’t really hurt anything. It is sort of like putting a floor polish on metal.