Not sure how you can quantify darkness on its own... it's just the lack of light. Your article explicitly says this numerous times. I suppose you could say "darkness" has a speed but it's simply the speed of light as darkness is just what is left behind after the light is gone.
Quote (thorsjsp @ Apr 16 2014 07:15pm)
we wouldn't be seeing the darkness in real time.
Sure you would, light or lack of light is simply an observation. Your always going to be looking from your own point of reference, it doesn't matter if your observation will be different in the future your still seeing "darkness" now. However it's probably damn near impossible to find any part of the sky without some light in it even if it isn't in a spectrum our eyes can see.