I've thought the same thing as Phael. I'm sure there are plenty of writers who will not blurb something they dislike (or found mediocre), I'm sure there are plenty who will also look for something nice to say. I'm more likely to assume the latter, which is unfair, but it seems like the majority of people I've met in life will...embellish their opinions of something to keep the peace or gain whatever they gain from it. Not even necessarily lying, but pointing out something good, even if it was the only good thing they had to say about it. Granted, I'd also assume that there are good professional reasons not to do this as an author--namely if you keep recommending books that suck, people might think less of your opinion, though I don't know that it really matters that much.
Personally, I don't buy based on blurbs. They go into the same category for me as those little review quotes put on books or movies. I realized pretty early on that a movie that says, "Stunning" might be from a quote that says, "While the graphics are stunning, that's the only good thing about this movie." Or worse, the original quote might sarcastically say something like, "I walked into this movie thinking it would be a top contender for movie of the year, but after seeing it, I realize the only list it might end up on is
worst movie of the year." Then the quote on the trailer says "Movie of the year!"
In other words, I'm skeptical.