My pleasure, its great knowing how easy it is to get going :)
Hmm, I see what you mean with that screenshot. It looks like the darkness is made up of a grid of black boxes that dynamically shift their opacity based on the square next to them.
You could mimic that with a similar style but your right, it may not be efficient as its probably something they optimised the engine around. I could be wrong about that to be honest its not something I have played with too much.
On the other hand though, the underground shot above is easily achievable. With one tiled object set to 70 percent and a hand full of light sources. If you think within the box your creative problem then becomes how to transition from light to darkness when going underground.
You could have a triggered event that once you go below ground level your opacity could transition from light to darkness the deeper you went.
Just a quick thought anyway.