And The Blue Rose Companion is split as many ways as it's possible for a book to be split. It's basically just another 120 pages of the core. It has everything, and that's useful, but for the most part it doesn't go into depth. We learn there are vampires in Aldea, but nothing about specific vampires or the ways they are used in Blue Rose's vision of romantic fantasy. Likewise, ritual casting is introduced as a new magic mechanic, but we only get three specific rituals (by default, it works as a point buy system where you improve your regular Arcana by making their use inconvenient). Wellsprings, magical structures, new darkspawn types, skill tricks - all welcome, all shallow.
The only thing that really gets expanded treatment is character paths. Blue Rose has three generic character classes - Adepts, who specialize in magic, Experts, who specialize in skills, and Warriors, who specialize in combat, but each of those classes had a number of "paths," which are named like subclasses (the Healer Adept, the Bard Expert, the Swashbuckler Warrior, etc), but they were nothing more than suggested starting builds. The Blue Rose Companion expands the Paths to be full 20-level suggested builds, but they're still nothing more than character concepts. They're just lists of feats to be taken at every level, and you can easily swap feats in or out without changing much of anything. None of them have unique powers or mechanics, and so it's unclear why they actually exist (aside from helping players see that you can build a variety of characters with the Blue Rose system).
On the balance, paths are fine, but then each of the 20 paths gets two and a half pages in Chapter 1, where we learn about their background, role in society, and religious affiliations. And there's a lot of good worldbuilding stuff there, but Paths are less real even than Classes, so how much healing magic do you need in order to identify as a Healer? Like, it's good to see a bunch of different educational and career paths that exist in the world of Aldea, but most players are going to make in-between characters, that combine traits from multiple paths or veer off in an entirely unexpected direction. So what is this for, exactly?
Overall, though, I'm pretty happy with The Blue Rose Companion. The largest part of the book was maybe too much wordcount for too tenuous a subject, but nearly everything in here was useful. If I ever play Blue Rose again, it will be an essential supplement.
Ukss Contribution: As much as I didn't care for Paths as a concept, my favorite thing from the book came from one of the path descriptions. In the "Infiltrator" path, when it talked about non-human Infiltrators, it suggested, "rhy-horses can and do masquerade as mundane horses, allowing them to pass virtually unnoticed by most people." The thought of a talking horse spying on humans on behalf of a horse government amuses me greatly.