Would you post some more photos of the side and back of the connector.
There is a possibility that the contacts and wire terminations we're "repaired" at some point.
Also your "black" wire seems to be brown about where your fingers are holding the wires.
Just to confirm... You traced out the "black" wire all the way to a body ground?
Also I agree that the power side of a fixture should NOT be attached to the base of a fixture, in your case if the bulb holder popped out and contacted body ground it would short.
I would get a replacement socket and rewire it properly.
