Unity doesn't support 'negative light.' There are a number of posts on the forum talking about different ways to get the same effect, but since you're trying to bake the color in they may or may not help you. Just a thought: could you bake the scene, then add your cookie and have it project positive / normal light (which you then bake to a second series of images), and then do the darkening in Photoshop? Just do a difference on the baked lightmap(s) containing your cookie vs the original lightmaps without, and then apply the difference as a subtraction to your original lightmap - presto, instant darkness.