It's just a normal usage of the word illuminate, i.e. to "make (something) visible or bright by shining light on it; light up." It means that creatures and objects within the darkness can't be seen, that the area remains heavily obscured for the duration, as long as no magical light greater than 2nd level comes into contact with it.
I’m not talking about attempts to hide though, I’m talking about line of sight and cover. You kind of need to know what can be seen from specific vantages to establish that, don’t you?
If you are standing in conceiling terrain, you are able to hide from anyone in any direction - as you are in the terrain. And if you are behind Blocking Terrain then you have already established a specific vantage. Otherwise the word 'behind' has no meaning.
Wha...? It’s like you didn’t even read the post you’re responding to...
He's probably just reinforcing the idea I've tried to convey. Back in 4e, things like "Concealing Terrain, Block Terrain, Bloodrock, etc" were real game terms, codified in the rules. Every terrain feature like stairs and arrow slits also had their specific mechanical effects specified in the rules. "How it looks like" were never really an issue.
Really? I think it's been described many times. I'm not sure why you have trouble imagining what darkness looks like. It looks dark. Everything in it looks dark. Dark enough to be heavily obscured (IE to be really hard to see). The only difference between the ink-blot idea and this is that "our side" (as you call it) has it looking like darkness rather than looking like some kind of opaque-black-sphere. It's dark in the area. I'm not sure why that's so hard to imagine.
So the people standing outside the radius of Darkness can see you, just not enough to get the mechanical benefits? I can see that. Like a field of flickering anti-light that reveals distorted glimpses of the things inside. And being Hidden inside it means they don't even see you in there.
You got it! They can sort-of see you. If you hide, you are gone. If you don't, they can see movement, or a shadow, or a silhouette, or whatever seems the most appropriate to the story being told at the time. Enough to, say, shoot arrows at you, but with a good chance that they'll miss.
Because that sort of darkness can only render things invisible if there is not an illuminated area behind it! If the spell is cast in a bright area, it either has to render the creatures in it visible as silhouettes, or if it creates some sort of obscurement on the whole area, and that obscurement must also affect things behind it! There literally are no other option that are imaginable.
