Well, unless I'm tripping, the Grand Fountain is made of pure darkness, which is why every darkner fits in with the dark world.
So I was just thinking because Ralsei is a denizen of the Grand Fountain dark world (and always has been) he, too is made of pure darkness?
Another theory I have is that the Grand Fountain can make darkners into pure darkness over time, thus allowing them to be in any given dark world without turning to stone. Like its influence changes the darkners within its dark world to be darker or something?
I do agree that pure darkness would be darkness without any one will (determination perhaps) imposed on it.