This the tach terminal wiring.
The black wire seems to be a loose cover - like heat-shrink tubing.
My guess is, this covering is over a White/red wire, that goes to the over-rev limit box.
That would explain both of the (conflicting) wiring diagrams. (one showing black, the other showing Wht/red).
Of course, I still don't know why the tach isn't functioning. I'll look at the CDI ground path next.