The Dark mode setting is probably unlocked after completion right?

And does anyone know if there are any special armors for playing on Death march?

I was wondering the same thing. Anyone know the answer?

As far as I understand there is no dark difficulty, instead it has been integrated into the regular difficulties (in the previous games it came out later because of the enchanted update).
From what I read, after finishing the game once you unlock the Hardcore difficulty which is permadeath.
Hope I could help.

So they have an achievement to finish the game on Dark difficulty, but there is no difficulty named Dark? WTH? :D

Just the story - Easy
Story and Sword - Normal
Blood and Broken Bones - Hard
Death March - Dark

I'm talking about the permadeath mode, because that is totally not Death March...

That was Insane difficulty level.
Dark = Death March.
No Insane in Witcher 3 (at least yet)