Japanese books say to leave the nakago (the tang of the sword) to age, because the condition of the nakago indicates how old it is.
I think this is good advice for the mid-1600s. But we are past the Edo period now. Swords are historical treasures. If we continue to let nakago “age gracefully” then there is a future where they turn into dust.
Not now, not next century, but not so far past that.
The black oxidated state of nakago are fairly stable, but not perfectly stable. Otherwise, there would be no “graceful aging” at all. Logically, we cannot have it both ways, that the nakago is safe from eroding away and that different period nakago will show different aging conditions. If these nakago were not slowly rotting away on a centuries timescale, then they would hit a stable point and then never change.