Bleach's Soul Reapers feel like ancient folklore. The word shinigami is a literary invention younger than the novel Frankenstein, and its most famous version was translated from the Brothers Grimm.
Documented
The paper trail starts in 1706
The word shinigami does not appear in Japan's classical literature. Folklore scholarship tracks its first sightings to Chikamatsu Monzaemon's puppet theater: Shinchuu Nimai Soushi (1706) speaks of the road the god of death leads down, and The Love Suicides at Amijima (1720) of one possessed by a god of death, a writer's pun on paper, kami, and god, kami. The Ehon Hyaku Monogatari of 1841 gives the shinigami as a possessing spirit of the dead that drives the living to repeat old murders and suicides. And the best-known shinigami tale, Sanyutei Encho's rakugo Shinigami, is an adaptation of the Italian opera Crispino e la comare, itself kin to the Grimm tale Godfather Death.
Assessment: Fully documented in the literary record. The shinigami of tradition is a theatrical ghost of possession, not a soul-guiding professional, and the friendly death god with a human personality is a postwar graft from Western fiction.
Stories
The ancient-god assumption
Everyone, Japanese audiences included, assumes the death god is as old as the kami. The folk version folds Izanami, who gave humans death, and King Yama, the Buddhist judge of the dead, into one figure and calls it shinigami, a single tradition from myth to manga. Bleach itself leans on this assumption: the Soul Society is drawn as an Edo-period Japan that feels timeless, so its death gods inherit the illusion of antiquity.
Assessment: The assumption is the interesting artifact. Japan's actual folklore had death spirits without a death god; the unified figure was assembled later, and pop culture finished the job so thoroughly that the seams vanished.
Interesting claims
The Western transplant
The sharper scholarly claim: the shinigami as a personified, human-feeling death, the kind Ryuk and Rukia both descend from, entered Japan only after the war, with Western fiction. On this reading Bleach is not drawing on Japanese tradition at all; it is re-importing the Grim Reaper, dressing him in kimono and kanji, and selling him back as the most Japanese thing on television.
Assessment: The timeline supports it: the classical record is thin, the Grimm-derived rakugo is documented, and the modern archetype is postwar. The twist the claim understates is the genuinely Japanese part: Bleach's death gods are salaried bureaucrats in a celestial civil service, which is a satire no Western reaper ever carried.
Sources and further reading
Chikamatsu Monzaemon, Shinchuu Nimai Soushi (1706), the road the god of death leads
Chikamatsu Monzaemon, The Love Suicides at Amijima (1720), possessed by a god of death
Ehon Hyaku Monogatari (1841), the shinigami as possessing spirit of the dead
Sanyutei Encho's rakugo Shinigami, adapted from Crispino e la comare and Grimm's Godfather Death
Kitamura Masahiro, Shinigami no Meruhen: Grimm tales and Japanese rakugo, Sundai Forum 18 (2000)
Tada Katsumi (1997) and Kimura Fumiteru (2007), the death-god question in Japanese religion
Bleach, chapter 1: the death god as celestial bureaucrat
Worth asking
If the most Japanese of gods is a Meiji-era translation wearing a kimono, what does that make the religion you thought you were watching?