One glaring thing i noticed that people have missed is that Yasuke had no last name (family name). In ancient times, Japanese peasants didn't have last names only the nobility and the samurai did. So the fact that Yasuke has no last name/family name and was never given one means he was never accepted as a samurai. (besides the fact that you know....the title of samurai was passed down through blood and you couldn't just become one anyways)
Of course this is just if Yasuke did exist, but he didn't.
There were periods where a peasant could become a samurai, but he would be given a family name afterwards. This would be a blood soaked era when they needed all the warriors they could get. Most would be sent back to being a peasant. There are forms of Karate entirely from those peasants who wanted to continue teaching how to fight.
Of course there was a black market trade for names. Seven Samurai had a character who did that. The gambling halls and whorehouses often bought names for themselves. It's part of the history of the Yakuza.
That being said, there was no way Yasuke the black samurai existed. Catholic monks are well known for straight up lying wherever they were. I read so many stories of how the European settlers arrived on a day that was to celebrate a god, and somehow the natives all thought these new men must be the gods that it's now a tell for when someone hasn't done their research for history. It all sounds like nonsense.