You made me curious, so I scanned through my copy of the National Electrical Code. The only color codes they define are for grounded conductors (article 200.6). There is no code related to coloring of ungrounded conductors.

My best guess as to why this de-facto standard came to be is that originally all insulted wiring in houses was black (see Knob and Tube Wiring). They didn't care which wire was hot or neutral as long as there was 120v nominal difference between them. This was generally a bad idea, because they were allowed to switch the neutral wire to kill a circuit, which meant that when you changed a light bulb the metal shield at the base of the bulb could still be hot even though the switch was turned off. I suppose that when they decided to differentiate hot from neutral they had to assume that all black wiring in existing homes was potentially hot. So they chose the polar opposite color (white) to signify that the wire was neutral.

That's the best I can come up with. And this applies to wiring in The United States only.