I really wanted to rate this one higher, because so much of it is so good. The world that's set up in the first half, to two-thirds of Bloom is both unique and approachable: a solar system rotten with alien spores that have driven mankind to the outer planets, cities with immune systems, a cult that seems enthralled with the very alien force that's evicted humans from the inner planets--it's a great setting. But after a certain point, pretty far in, I started to realize there felt like an awful lot of plot to get through, in a rapidly-diminishing number of pages.
I equate the experience of reading Bloom with trying to get somewhere by climbing over a tall hill. The trip to the top takes a long time, but it's a nice hill, lots of scenery and wildlife, to make the trek quite enjoyable. Then, you trip on a tree root, and start tumbling down the other side. Whatever sights there might've been whiz by in a bewildering rush, until you suddenly hit the bottom and come to a stop. And I do mean suddenly.
You know how in sci-fi movies from the '50s, the army or the scientists would blow up the monster, and "THE END" would immediately come up on screen? The ending of Bloom is kinda like that, except it doesn't even go out with a bang. After heading out on their journey and being chased through half the solar system by mysterious ships, there's a span of about 20 pages, where a shocking discovery is made, the protagonists face a choice that could change the fate of humanity forever, the true nature of the alien spores is revealed... and then things just sort of trail off, with not much accomplished at all.
If the final act hadn't been such a jumbled mess, I could easily have seen myself rating Bloom a four, if not a five. But it's a perfect example of how a bad/rushed/lackluster ending can ruin a book. And boy, does the last act feel rushed.
Oh, and the book jacket lies to you: It says very clearly that the mycora (spores) are man-made, when every indication throughout the book itself suggests they're alien in origin. That's just sloppy.