It is important you find where the battery found ground. To do that, and involve the blue wire, the B+ wire must have contacted the alternator frame, if the alternator did not fault internally. That can happen by rubbing through on an edge, or sometimes the ring lug breaks (vibration + time) allowing the lead to "weld" itself to the frame. You have to look closely for witness marks, and dissect that blue wire to assure yourself it was heated from the outside and not the inside.

If you never do conclusively find the root cause, the only remedy I could think safe would be a replacement of both harness and alternator. That could run something like one of your neighbor's car payments, I suppose.
--
Art Benstein near Baltimore

Mistakes are the portals of discovery. -James Joyce