Well, we've seen the GM's interpretation and I'm not going to argue with it. I just want to know for future reference.
I can't find anything in the FAQ or Rules of the Game, and the spell description itself isn't very helpful. All it says is that normal lights and light spells are incapable of brightening the area, but it doesn't say anything about existing darkness. It's supposed to 'radiate shadowy illumination', which to me sounds like 'radiating anti-light', as opposed to 'making everything in the area shadowy'. So I would think it would add to (not replace) existing darkness. Because, y'know, it would be weird to have a darkness spell make an area brighter.