I think it might be a fuseable link (or should be). We're going to follow that black wire from the RR all the way to the ignition tonight. I'm almost certain there's a fault in it. Here's why. About an hour ago I fired up the bike and check the volts at idle. 12.5-13.5. I ramped up the RPMs and I couldn't get the volts passed 14.8, which according to the chart is where the RR should start dumping to ground. I then checked the black wire and the voltage had dropped to .7 and lower. 30 minutes later I did the same tests with my wife watching and the charging volts went over 15v at high rmp and the black wire was up over 1 volt again. That tells me there's some kind of fault in that line. Unless the RR/VR can fail in a funky manner and cause that but if I'm reading the wiring diagram correctly the black is carrying current TO the RR/VR not away.
Anyway we're draining the fuel out so we can remove the tank and chase that wire