Rob, that does look like a bit of a mystery. I guess the smart Alec answer is plug the whole lot to ground and see what blows up!
Can you confirm your model and if the car is RHD; i.e. passenger side is the left side of the car? Do you just have a single two fuse box under the bonnet or the more elaborate set-up?
As stated, Black is typically ground in the loom runs, although some component wires (like the heater motor, radio, window motors, etc.) may be black to where they join the loom. Can you check the wire and see if there is no resistance to ground?
I checked the wiring diagram for my '69 Federal LHD, and there does not appear to be any legitimate black grounds going through the firewall to the engine bay (except maybe grounds associated with the headlight microswitches located in the dash on the Federal cars, which is a whole different deal). This makes sense as in stock form as there is a grounding bobbin on the left side of the engine bay, and another ground bobbin on the right body mounting bolt just behind the dash mounting bracket. I am going off of my Federal diagram because it shows the physical location of the connectors rather than purely schematic diagram. If the grouping of black wires do in fact go through the firewall I might suspect PO ran extra grounds, but this is completely a guess.
When you mention the 'fan stat', do you mean the radiator mounted Otter switch sensing coolant temperature for the engine electric fan? If so, I can't help on that one as I just noticed it is not included on the Federal wiring diagram and the entire section K Cooling is missing from my Workshop Manual.
Good luck with it. Should be able to figure it out with a bit more info.