I've got a deadline for a paper tomorrow at noon, so here's the short version:

For what it's worth, the one-drop rule came quite a bit after the elaborate system of classifications –– it certainly wasn't law until the early 1900s, and prior to that you had to have at least a quarter African ancestry to be considered mixed race.

Categorizations themselves largely seem to have come out of the Enlightenment-era culture of classification and renewed interest in the body and how it could be mobilized in a variety of ways... physically, socially, ideologically. What popularized them to a large degree was slave-selling and human marketplaces as a means of describing the colour of individuals, very similar to how we might describe the coloring of horses. Do you know the difference between a bay horse and a chestnut horse? Do you know the difference between a dark bay and liver bay horse? How about a sorrel chestnut, or a pangere chestnut? Some of these horse colours are descriptions, and some are thought (or confirmed) have some sort of genetic factor. Most of us don't care: it's a horse, dammit. In the days of ye olde biological racism, it was somewhat similar; there were all these not-clearly-defined ideas of who what described as what and why, some of it based on psuedo-science, some of it based on appearances, and ultimately it faded out of use because eventually as a society we didn't need it. By the end of WW2 most societies condemned the concept of biological race (even if they had been practicing it very recently) and so in a way there's even a loss of identity there for mixed-race people who today might feel they're caught between the worlds. That's a whole other story, though.

That said, even earlier versions of the "one-drop rule" weren't necessarily a hard rule or even upheld half the time. There were occasions where mixed-race individuals sued the courts for their freedom but generally could be understood as a plea "for their whiteness", usually to say they were deserving of their freedom due to the fact that they were more white than black, regardless of heritage etc. Elizabeth Key was the first to do this, and many others followed.