It may be due to my relative lack of knowledge when it comes to how any of this actually works, but I'm with ruskul on this. It makes more sense to me to implement a light-caster, rather than a shadow-caster. Where everything is in shadow, unless it is hit with light. I hope this kind of thing is technically feasible, as I am running into a few issues while trying to implement a solid lighting system that makes sense to me.
That said, the current shadows are really good at what they do, and are a million times better than nothing at all. Ideally I would only want to use sprites (light-blobs) for coronas or baked in static lighting, rather than using them for lighting the entire world. Or at least, have the ability to cast point light and light cones with fade off and whatnot to have a more solid interactive lighting system. I think the limitation of having to create separate layers for lights is what is causing me the most trouble.
Some examples of what I've hacked together. You might be able to see where this would be improved if the lighting system didn't work backwards, as it were.