Dude, the Black Hair can mean 2 things:
1- a temporary personality change that will culminate on a cool moment where Sanji will overcome it by reinforcing his own principles and emotions. That's cool.
2- a mere visual change. Which I also like.
I know some people are kinda jumpy about this "modification" theme, but I assure that basically no one defends a complete change of Sanji's character just for the sake of changing it.
Every hero needs a moment of overcoming ordeals.
And about "what makes Sanji unique", we are waaaaaaaay past that. What's unique about Sanji unlocking haki that every other relevant enemy/ally already have at this point? Making him a cool Power Ranger with cool abilities>>>>>>>>>>>>>>haki bloom number "whatever".
We are getting a good setup for Sanji to actually have an impact at the EoS, asides from simply finding the All Blue, which is still gonna happen. We got an Admiral that seems custom made for Sanji, we are gonna have connections with Vegapunk, one of the most hyped characters in the entire story, etc. Oda is elevating the character back to his spot as a Monster Trio member.
But I guess y'all prefer the good old days of vergo cracking his leg, getting rekt by Doffy, not even getting a 1v1 on FI, not getting a 1v1 on an arc dedicated to him, huh?
Well, excuse me if I want to see my favorite character tackling new challenges, both battle wise, but also inner ones. And excuse me for wanting to see a Black Hair mode that would be a neat reference to Dragon Ball.
Black Hair happening or not, this being my "ideal" Sanji development that I've always dreamed about or not, I'm gonna enjoy this moment, hoping that Oda will keep delivering something that I at least find "good". Especially when this arc has been trying so freaking hard being an Water7/Ennies Lobby 2.0, just like Dressrosa tried to be Alabasta 2.0 and both failed to do so.