I picked up Van Richten's Guide to Ravenloft and have been reading it cover to cover. This is my review of the book, chapter by chapter. Chapter 3 covers over 30 domains, so I'm posting about it as I work my way through the different domains.
All I knew about Bluetspur prior to reading this section was that it's an uninhabitable domain ruled over by a god-brain. It had no settlements and no apparent citizens. In one supplement, it was revealed that its darklord was an ilithid god-brain, but I don't recall that ever being explained beyond that phrase.
Well, there's not that much more to say about Bluetspur, as it turns out, at least in the same sense that in Starfinder's Pact Worlds source book there's not much to say about Aucturn or Akiton. It's desolate, deadly, and empty.
And yet the authors find interesting and inspiring ideas for players to explore. Mount Makar is a mountain that's not really a mountain, with a significant purpose that I won't spoil here. The Citadel Subterrene is a subterranean city consisting of pods or nodes in which ilithids dwell.
And then there's the god-brain.
The darklord of Bluetspur is an ilithid god-brain. A god-brain traditionally binds all ilithids within a colony together, acting as a kind of knowledge and psionic coordination repository. This god-brain betrayed a bunch of other god-brains, and so they had to destroy it. Unfortunately for the wanderers of the mists, the Dark Powers intercepted the god-brain on its way to whatever hell was waiting for it, and gave it a domain of dread all its own.
Admittedly, so far Bluetspur sounds essentially like just another Underdark. But there's a slight twist, I think, because it's implied that this god-brain actually wants to be put out of its misery. It's implied that it isn't, unfortunately, so self aware that it understands that it wants to be destroyed, so I get the impression that it will definitely fight back. However, the idea of a colony of ilithids that may want to both help and hinder players from destroying a self-loathing god brain sounds pretty great.
I think I'd probably add some third party into the equation, even if it were an ancient third party that isn't even around any more. Maybe the ilithids are dwelling in an old city constructed originally by gnomes (it actually sounds a little like Mount Nevermind in layout, minus the gnomeflinger). It could be part of their dark domain curse that they have to put up with mis-sized equipment. This ancient civilization could serve as a message from the past, urging the players toward their ultimate goal. Or maybe there are Gnomes still in Bluetspur, in hiding, looking to help the player characters destroy the evil overlord. Or you could just have the players stealth around for the entire adventure, encountering no one but ilithids.
Like Strahd, the god-brain darklord doesn't get a stat block. In fact I don't even think the book mentions where to find stats for an elder brain. Luckily, I've read Volo's Guide to Monsters so I have stats for a CR 14 ilithid elder brain, complete with options for its lair. I do find it odd and very disappointing that stat blocks apparently aren't considered important for the darklords. If the earlier chapters hadn't specifically encouraged you to feature the darklords prominently in your adventures, I don't think I'd mind so much, but as written the book says one thing and then fails to provide you with the tools to follow through. At least with Strahd you can slot in a standard vampire stat block from the Monster's Manual, but what are you supposed to use as a god-brain if all you have are the core books?
However you run an adventure, I think Bluetspur could be a fun, forbidden planet, cosmic horror experience. Unlike the Underdark, the player characters can't just decide to tunnel up and to the right until they get back to familiar territory. This is a domain of dread. You have no option but to play the game to the finish.