Jim - An old electrician told me about the black being hot when I was a kid. Yes, we did have electricity back then.
Taz, yes that was necessary since you were quoting electrical codes, obviously YOU had Jeeps and buildings mixed up. If you knew the difference between a Jeep and a building, why did you bother to show building codes?
The poster has a Jeep to put the fan in, not a building. Look at post #1.
Black as ground - Not just battery operated devices, TVs, radios, computers, aircraft, spacecraft, industrial machines, etc. often use black to signify a ground or return.
Using a battery does not change Ohms Law for the rest of the world.
Permanent magnets? Maybe so, I didn't look to see how old they were. You'd rather have something more complicated? (on a Jeep!)