Hardening is the prerequisite to barrier/adv CoA. You harden and then let it flow externally. Whether your body/arm stays hardened/black once you have created the barrier is irrelevant and secondary.
Hyo using barrier
Luffy using penetration.
Luffy using penetration for first time
Rayleigh flashback
Hyo explaining both advanced types
And what underlines the succession of phases in creating barrier, Zoro´s arms are black, and then the swords, but then his arms are not anymore