What the heck is this?

Ok so I couldn't resist candling again today to see what's going on with my BO egg. It has this really dark mass in it... Almost looks like someone made an inkspot on the egg but on the inside of the egg. It has definitely gotten darker inside than it was a few days ago when I first saw the mass but not near as dark as my other eggs and the darkness almost looks slightly translucent still. I do think I saw some movement though. Any ideas on what the heck could be going on in there?