Hey Eric- just changed my blower so this is still fresh. The line ahead of the transfer box is energized by the genset running (on SM 422). The hot line goes fuse, capacitor, then fan. I'm not sure about your black wire.....if that's a 4th wire coming from your fan, we don't have one so can't help there.
As an aside, due to charger overheating, we installed Hella 24 volt fan in ceiling light block over genset, pointed at charger. Switch is by watermaker panel. It resolved my charger overheating issue where new blower had made no difference.
Tom and Kirstin