Regular readers know that
So when you have Emily posting on an illusion dealing with Saturn's rings, you know you're onto something pretty cool.
See how the ring looks darker where it crosses the moon Mimas than when you see it over a dark background? Emily checked, and the ring pixel values are actually higher there!
I suspect Emily's conclusion is correct; light from Mimas is coming through the ring, adding to its pixel value. But the rest of Mimas is so bright that the ring looks dark compared it it. That's an odd illusion, but a well-known one.