I found a stock 500-C, the one I used to create a PHOTOFACT, and there is no black wire from to the rear outlet from the 3-way t-strip middle terminal. See attached image. Since the orange wire connects to that same rear outlet terminal as the black wire, I see no reason to keep the black wire even though the black wire connects to the white wire which theoretically serves the same function as the orange wire - to switch on something.
I'm trying to imagine why one would add that black wire. I can only surmise that something went wrong with the orange wire at the switch and rather than risk hot soldering a fragile switch, the person added the black wire. But eventually the orange wire was reattached to the switch. Also, the fuse terminals look like they've been messed with. Maybe the fuse wiring was involved. Anyway, it doesn't matter.
I believe I know enough to go ahead with the relay install. I've attached my final wiring diagram. It's the same as what Tom suggested accept I want to keep the brown wire attached to the switch so I swapped what connects to relay terminal A2, - instead of + so to speak, and the orange wire goes to a different terminal on a rear outlet. Feedback welcome.
