The black wires are leads from the ignition coil (in the picture). The 40 year old lead wires on the coil started to deteriorate so I replaced them with the only thing I had at the time - black! Nothing DC has a negative side grounded to the frame. It's all tied to the negative terminal on the regulator.
All the A/C stuff on the bike (that isn't connected to the regulator) is tied to frame ground. Anything supplied with DC positive is tied back to DC negative on the regulator.
The "floating ground" for the 12v coil is the electrical washer thing coming from the coil that the stator screws go through. (edited)