Look I don't know much about the context, aside from some game adding a black samurai or something, but I can tell you I would be just as pissed with a white one.
Fun fact, these are both real people from ancient Japanese history (and pretty famous ones at that), but you can guess which one the "gamers" are upset about.
Some more details because these two are really cool and interesting:
The white guy is a British navigator from a Dutch voyage to find a new route to India to steal control of the spice trade from the Spanish and Portugese, whose ship ended up in Japan after almost being destroyed in a storm. His name was William Adams, but the Japanese couldn't pronounce it and called him Anjin-san instead (literally Mr. Pilot). He was made a samurai and given land for his help in a war of succession, as well as his aid in founding the Dutch/British trading relationship in Japan. He lived out the rest of his life in Japan and his house can still be found today as a historical site.
The black guy, whose history is a lot less clear, was named Yasuke. He was most likely a slave originally from Africa, but there are no real records beyond that he was brought to Japan by the Jesuits, who were the sole foreigners allowed into Japan at the time. He was presented to a famous lord by the name of Oda Nobunaga (one of the most famous people in Japanese history), who was so taken with the man that he took him into his service as a retainer and samurai and gave him the name Yasuke. Yasuke served Nobunaga up to and during the betrayal that led to Nobunaga's death, after which Yasuke disappeared from historical records.
Wait, people are losing their shit over the historical accuracy of Assassin's Creed ? Did they miss the part where the entire franchise revolves around some DNA magic, or the "apples of Eden" stuff ? Or whoever Borgia pope with literal magical powers we fought in AC2 ?
AC has always been about (somewhat loose) historical accuracy of the setting, ie, places, buildings, culture, etc. They take tons of liberties with characters and "realism"
... It's a valid complaint to ask that a historical African man be represented as a Japanese man? How?
Do you mean they should have used literally any other historical figure or even a fictional character, as long as it was a Japanese person? There's a second playable character who is Japanese.
I don't think there's any shortage of Japanese samurai protagonists in games. From a representation standpoint I don't think there's any issue... And Yasuke is an interesting character that's worth exploring. There's enough mystery to his story that he fits perfectly into an Assassin's Creed game imo.