Hi.
In fact, we had discussed multiple times Kajuku's example and I even said in my first post about it:
To summarize: in a black and white manga you can draw black a lot of colors...bar one, because for white colored object you use...white lol.
Anime did not draw Ryo's blade green, brown, red or any other color that in theory could be represented as black, as whit Kajuku, but white, which is the only color it doesn't make sense to use.
That's why I see the problem.