there is many tools within nuke to achive that, try the furnace degrain or denoise with strong values. they will kind of averaging the pixels a bit more smartly than a simple blur. Contrasting your edges with a sharpen before doing it can also helps. keying the luminance and grading the dark area through that also smooth details. a kronos or similar without changing the time can also smooth details....
mix different things together. try to change the filtering methods of these tools too, the result is different.
but blur and mix always come back. It is often a matter of smartly regrain these area that make it work
cheers