First of all, the surrounding sources of radiation help determine the color. If there are no surrounding events emitting light of the blue wavelength, then R2's head would not have that blue wavelength to reflect (it would appear black). In terms of the planet or the death star, they probably wouldn't reflect enough light compared with the solar system's star(s) to help alter a normally blue surface's color. Even if they somehow did, R2 would not be black in that instance though. Now, if they were on the dark side of one of the objects, and it effectively blocked all visible light equally, then the remaining ambient light would leave R2 blue, but dimly blue. If either object were to block the blue wavelength more than other wavelengths, then R2's headpiece would look black compared to the light reflecting off of other objects. None of that is happening in any of the movies as far as I can see though.
So what you are saying is... 
