It can be true that there was a black samurai without there being *many* black samurai. It can be true that slavery existed in Japan without it being what Western audiences think of as slavery. (See my thread on this from two years ago!)
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Dr. Paula R. Curtis
@paularcurtis
Premodern historian here. No. Slavery absolutely existed in Japan for centuries. There were various forms of unfreedom experienced by Japanese as well as Chinese, Koreans, and likely others in various time periods. Bondage that was linked to debt, that was hereditary, etc. 
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