Teeth blackening in Japan is also referred to as Ohaguro, which is also a name given to the dye itself in some sources. While the tradition is recorded as early as the first century CE, the origins of the practice are unknown and some suggest they may be prehistoric.
Marc Geissberger writes in Esthetic Dentistry in Clinical Practice that this tradition “became popular among married women as a way of distinguishing themselves from unmarried women and providing contrast to their white painted faces.” He also explains that the practice caught on, only briefly, with the male Samurai class as “a symbol of loyalty to one master within a lifetime.”
Geissberger says of the dye that it was “an oxidized mixture of iron shavings and powdered gallnuts.”
Lewis, Memory, and Elvin-Lewis write in Medical Botany: Plants affecting human health that the practice was so widespread that by 25CE Chinese documents refer to Japan as “the country of the black-teethed people.” They further indicate that the process of blackening was part of daily beauty regimens for Japanese wives: young women especially. Older women would reapply the dye as needed for leaving the house. They report that black teeth “were taken as a sign of fidelity and were considered beautiful until European distaste for their appearance became known.” They also state that scientific consensus shows that the blackening had some dental benefits: it helped to protect against enamel decay by acting as a rudimentary dental sealant, and helped to disinfect the mouth against cavity-causing bacteria.
Lewis et. Al. describe the formula for Ohaguro as follows:
Well-boiled tea to which were added nails or chipped iron heated until red hot, sugar, yeast, sake, and confections made from starch. This was allowed to age in a cool place in a tightly stoppered bottle for several months until it turned into a brownish black and yielded an unpleasant odor due to ferric hydroxide, ferrous carbonate, and ferric sulfate. It was assumed that the staining solution of the ferric tannate was formed on mixing with the tannic acid solution.
While it may have provided protection against enamel decay, the process was very unpleasant. Angus Trumble writes in A Brief History of the Smile that “the bitter taste and black stains of iron oxide must have crept into every corner of the mouth and spoiled, or at least radically altered, all but the strongest flavors, making a misery of eating or drinking.” He notes that tongue-scraping was common to help minimize the dye’s effect on the palate.
Generally, there are a vast number of theories as to how or why the practice emerged, and what it symbolized. Trumble offers two possible explanations. “Ohaguro originated in the Buddhist idea that white teeth reveal the animal nature of men and women and that the civilized person should conceal them.” He doesn’t provide any source on this claim, and later in the same paragraph hints that he doesn’t place much stock in it himself. His preferred explanation, which he considers more “pragmatic” and “historical” (yet also, it should be noted, unsourced), is that “Ohaguro was first adopted in the house-holds of samurai warriors as a measure to protect their wives and daughters from being kidnapped or raped by their enemies. In other words, the teeth were originally painted black to look rotten and, therefore, repellent.” Without any credible sources, both of these theories may be just blind speculation on Trumble’s part, possibly not a little biased by our own Western preference for white teeth as beautiful.
Trumble brings us, perhaps unwittingly, to an important point. Attempting to explain Ohaguro is attempting to explain a cultural practice with origins lost to historical record and which lasted for centuries. The meaning that one group of people at one time may have given to the practice may be the reverse of what another group at another time understood the practice to mean.
The earliest source I could find from 1910 is by Henry Lovejoy Ambler in Around the World Dentistry, where he notes, “the present Empress set the example of discontinuing the practice.” It was already on the way out in Japan by the late 19th century.
We must be incredibly careful when reading about Ohaguro that we aren’t retroactively reading meanings into the record that make sense to us now.
EDIT: Formatting
While I have no on hand sources for the Heian period or its original conception, this practice carried on past the Edo period and was specifically commented on by Jun'ichirō Tanizaki (July 24th 1886–July 30th 1965) in an essay titled In Praise of Shadows (1933), which should at least give a flavorful insight into the desire of such an aesthetic by a Japanese source.
"Our ancestors cut off the brightness on the land from above and created a world of shadows, and far in the depths of it they places woman, marking her the whitest of beings. If whiteness was to be indispensable to supreme beauty, then for us there was no other way, nor do I find this objectionable. The white races are fair-haired, but our hair is dark; so nature taught us the laws of darkness, which we instinctively used to turn a yellow skin white. I have spoken of the practice of blackening teeth, but was not the shaving of eyebrows also a device to make the white face stand out? What fascinates me most of all, however, is that green, iridescent lipstick, so rarely used today even by Kyoto geisha. One can guess nothing of its power unless one imagines it in the low, unsteady light of a candle. The woman of old was made to hide the red of her mouth under green-black lipstick, to put shimmering ornaments in her hair; and so the last trace of color was taken from her rich skin. I know of nothing whiter than the face of a young girl in the wavering shadow of a lantern, her teeth now and then as she smiles shining a lacquered black through lips like elfin fires. It is whiter than the whitest white woman I can imagine. The whiteness of the white woman is clear, tangible, familiar, it is not this other-worldly whiteness. Perhaps the latter does not even exist. Perhaps it is only a mischievous trick of light and shadow, a thing of a moment only. But even so it is enough. We can ask for nothing more."
-Sourced from the Harper/Seidensticker translation, 1977.