Try
this link for DOHC wiring diagrams, and see if you can find something that fits. I don't recall having a "black box" on my 1979 model, but maybe something changed. If you're lucky, it may have a part number on it, with which you can search on bikebandit.com or the Google and see if there's more info.
Technically, everything should come back (eventually) to the negative post of the battery, be it frame, headlight ground, turn signal ground, engine (for spark plugs). Typically, there is a green wire that ties to the frame under the front part of the seat.
Good luck, and I hope this helps.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.