I would isolate the chick from the others so that they won't start pecking at it. Electrolytes are perfect for it. At this point, the only other thing you can do is make sure that it is not pasting up around it's vent (which would plug it up and kill it).
It is possible that it's umbillical was torn, and bled a bit internally. If that happened, then the chick could possibly recover once it's body removes all the excess blood under the skin (kind of like a mega bruise).
Unfortunately, if it's an internal organ that's bleeding instead, the kindest thing you can do is cull it. You'll just have to observe it for a day or so. If the dark patch fades, then it has a fighting chance.
Good luck!
