I think the blurb should be as carefully planed as the story itself.
Personally, I think they deserve more attention than the rest of the story, as no one will ever read your story if you have a poor description. Typos mean you (the author) aren't proud of your work, incomplete descriptions won't be a good indication of the story and meandering, unfocused sentences are a warning your entire story is badly written.
The key to a decent story description is to capture the feel and the main conflicts of the story. I'll provide a few examples of my own stories, rather than dumping on anyone unfortunate enough to get picked on.
The Cuckoo's Progeny (just published)
Al and Betty aren't like their classmates. They've developed peculiar skills which set them apart, and which sets them in a search for others like themselves. As they discover who they might be, they become alienated from friends, family and humanity in general. They must discover who they are, and where they belong, as there's no longer room for them in their old community. Between walking into disasters, ungrateful rescues and government agents tracking their every movement, they're searching for a way out and a way home, wherever that might be.
Searching out others like them, they build a crew to search for a ship which may take them to a home they've never known, to a reception they aren't sure will be welcoming. What's more, they have no one to rely on other than each other.
Here, the first paragraph lays out the various conflicts: isolated, disenfranchized teens, realizing you're different from everyone around you and they're attacked by everyone they encounter--the "paculiar skills" hints at the series title Not-Quite Human.
The final paragraph summarizes the plot (i.e. what they're trying to do, as well as what might happen in the future books).
The Zombie Leza: The Marriage of the Quick and the Undead
(My next, as-yet unpublished, book)
Leza's an unusual young woman. She lives with Zombies, protecting and caring for them. In an apocalyptic wasteland, where the living are few, she represents a unique hybrid, a living zombie. She speaks with the undead, and can teach the living how to survive. If they can only keep from killing her, that is.
Discovered during a final confrontation between a refuge of survivors and an oncoming horde of the undead, Leza steps up, saving the day. She represents more than just another day of survival, but whether that spells humanity's rescue or the future of the undead, is anyone's guess. She may spell redemption and rescue, or their ultimate annihilation.
Here, the "She lives with Zombies" establishes the main conflict, no human will trust her since she can call upon the combined might of all the zombies surrounding her, though she represents an unusual chance to learn more about how to control the zombie hordes.
The second paragraph summarizes the human's challenge, while she threatens their very existence, she also represents a chance of discovering a zombie cure.
Singularity (my currently posting story)
An experimental NASA flight goes horribly wrong, and the unlucky test pilot wakes up unhurt, back home. Struggling to discover what happened, Eric Morgan returns to NASA prepared to face a thousand questions as things continue to unravel around him. Is he still human or a new species, and what does that mean for those around him?
Eric encountered something strange in the dead of space which left him forever changed. Testifying before a Congressional subcommittee, he inspires fear and hope. As the danger mounts, he tries to fall off the grid and begin a new life as he struggles with what happened, but the rest of the world won't allow him to walk away. Is he a threat or a blessing? Is he helping humanity or threatening them? Should he explain himself or hide? And what about those close to him? How will the changes to him impact them?
Here, the first line contains the story's premise, while the second conveys the character's challenge. The third incorporates his main conflict: who has he become?
The second paragraph goes into more detail, outlining that he was altered in some way, and that the government (i.e. Congress) is out to get him, and that he's ultimately forced to runaway to escape everyone, presenting yet another challenge. It wraps up by summarizing how everyone views him after he's been changed.