Congratulations, you've found the mystery wire. Tom O'Malley's wiring diagram shows it connecting to nothing, and as far as I can see, he's right.
On my Spit, the Blue/Black connects to the driver seat pressure switch, with a plug-in connector, and through the switch to a black ground, as per the wiring diagram.
When I follow the Blue/Black wire back into the harness, though, it exits the firewall on the LH side, beside the fuse connection. I can't see that it connects to anything else. If I follow that wire harness across the firewall to the starter solenoid, guess what - I have a Blue/Black wire with a cutoff end that doesn't connect to anything too!
All I can figure is that it was meant to tie in to something in a later model? Or maybe its used in a California European version and not with US Federal. If you can find any more out, please post. There must be a story there.