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Yes, this is correct.
Hardened steel will etch much blacker than unhardened steel. Unhardened steel will at best turn dark grey (depending a bit on the alloys of course). No amount of etching can...

Your experience is wrong.
Harden half a piece of steel and leave the other half soft.
Etch that.
See what happens.

It is normal that soft steel etches a lot less, since the carbon is...

The spine has to harden as well, or the core steel will not etch on the spine.
And the point as well, or it won't etch properly either.
And those places are where delamination is likely.

In this case that wouldn't work, because the contrast layers wouldn't harden either.

I made another talon. Actually, this is the 3d. The second met an untimely demise when I decided to give water quenching a try. As it turns out, water quenching a suminagashi razor is not a terribly...