Guys, this is a feature of the "prevent black appearance" option for the lights. The thing is that it doesn't work in reflections or refractions. So your directional light will look white when viewed directly, but it will stay black when viewed in a mirror. This is not easily solvable because it's an unrealistic hack and because it's not exactly clear, in which situations the light should return white and when it should be invisible. For now, you can use a rayswitch material to circumvent this limitation.
Edit: So without seeing the scene and having more info, I would say that this has nothing to do with instancing!