Splicing to the Black won't hurt, but it's curious that the wire is missing. Some riders who removed this wire were simply de-powering the turn signals on the F2/F3/K7/K8 bikes, but it also turns off the instrument lights in so doing...
Check your tailight (if stock?) for the wire colors. If it has the BROWN and BROWN/WHITE wires with the GREEN 'ground' wire, your harness may have been using the instrument light circuit to power the running tail lamp, which appeared in some bikes near the end of production (leftover parts, maybe, or some from other models that fit, was Honda's practice in those days). This was more commonly found on some F0 bikes, but I think maybe the wire harnesses get swapped around in boneyard trades, sometimes? I recently fixed up a nice F1 that had a K5 harness in it, which looked this way.
I saw quite a few K6 bikes with F0 harnesses "in the day", which really confused us at out-of-the crate assembly times, since it was not yet in the manuals. I also know of some DOHC 750 harnesses appearing in the last F3 bikes: these were visible by a different-looking fuseholder on virgin F3 bikes (rectangular, same hole spacing). The wire colors on the fusebolck did not match the manuals.
The other 'missing link' you may run into is the "spliced wire" change on the power plug, which can affect the rest of the harness with a missing BROWN/WHITE wire: the Brown/White and Brown are spliced together in these harnesses, about 1" inside the black wrapping, and they run through the bike as Brown only. These appeared (occasionally) on the late F1 and early F2 bikes, the K8, and on the first year of DOHC750 bikes. They all use the same steering-head-mounted keyswitch and plug.
Now you know why I omitted the 1976-and-later wiring from my first book...
