Because in the manga we clearly see the hamon, and the VC painted it as black.
The anime Is simply another medium and
objectively what Is in the anime don't seem like a sword at all. It lacks the grease and the central line, other that being completly different from the manga and VC, which Is already telling.
Just look:
Other "White" swords in the anime:
It's like a sheath:
[IMG=ryooo]
https://i.ibb.co/yfBkx9g/ryooo.png[/IMG][/URL]
So, that Is the difference.
In the anime the thing doesn't even look like all the other swords
in the anime.
In the manga the sword looks exactly like a BB and was black even in VC (and official Databook):