Nitriding will not change the SURFACE FINISH other than darkening the colour. I am an Engine Reconditioner and specialise in performance engines, mostly high powered V8 engines for drag racing and oval track Sprint cars.
We get our forged steel cranks nitrided even if done by the factory first, it changes only the colour to a greyish hue, journals are polished again afterwards, just to make them shiny again, as they dull off after treatment.
Hope this helps, the treatment only goes a few tenths passed the surface.
Cheers.