I am a Cameroonian. I have a Russian visa, but I wish to stop at Belarus for a few days on my way to Russia. Will I need a separate Belarusian visa, or will the Russian visa I have permit me to spend a few days in Belarus?
A Russian visa does not exempt you from needing a Belarusian visa.
So yes, you will need it, although it won't be checked at the (open) border.