What are Unblemished?

The master of mana concepts inside the game, talks about Unblemished and Vim or some such, and makes it sound like a civilization, but I cannot find any mention of it anywhere else.

I've never played as them. I don't know.

Lore-based answer : the Unblemished are the worshippers of Sucellus after His rebirth as the God of Life.

In-game answer : it is a unique civ made by Tarq (the creator of Scion of Patria). Its unique concept is revolved around the "Vim". It was included in some patches ago, but cause unstability in multiplayer (?) thus being disabled at current condition. You can enable it manually but the civ is not "overhauled" yet to fit 1.42 mechanic.

I tried a for a bit. The Vim mechanic seems to be just another resource to manage. Is there anything to offset the fact that we have vim to look after? Like stronger units or equipment?

they have pretty badass dragons

and as hero a battlecaptain with pretty badass aura for his stack... and different kind of drakes... and a silver/diamond dragon. And a unique vim-mechanic which can give you gps and other boni or revolutions. Tarquelne, creator of the Scions, made them, but he isn't active anymore and iirc they still need to be adapted to the latest update... afaik, they are still playable if you enable them (change the 0 to 1, iirc).