Artistically speaking, you combine pigments of paint, or whatever medium you choose, and create black. But from what I'm reading, those who use this arguement actually have it backwards. You're not mixing these different pigments for their light reflective qualities. You're in fact combining them for their light absorbtion qualities. When you mix red with blue, you get purple. However, if you'll notice when you mix the same shades of these two coors, you get a darker pigment. That's because you've not only created a compound capable of reflecting blue and red at the same time, you're also absorbing these two spectrums based upon each individuals innate qualities. What you're doing when you're mixing these pigments is cancelling each other out until very little of the spectrum is being reflected.
Now, in regards to the arguement of whether or not it's possible to have a black lightsaber blade. According to descriptions of this admittedly fictitious weapon, it's emitting a blade of energy hot enough to cut through most any material, and thus it will glow with this intense energy. Something that is black is absorbing energy, and not emitting it. So a black blade could not be a 'lightsaber' because it would have to be absorbing energy and light instead of pushing it out. So what would happen is, theoretically speaking, if a blue saber and a 'black' saber were to cross, the blue lightsaber would be absorbed, at least partially, instead of being repelled by the 'black' blade. So instead of the 'black' blade stopping the blue blade, the blue one would simply pass right through.
It would be like a magnet for light and energy instead of being something that would act as a light-emitting weapon, as the description of a lightsaber entails.