It is common knowledge that the blurbs on the back covers of a newly published novels are problematic at best. Some authors seem to have a minor careers in writing blurbs and, like in Bull Durham, you’ve got to know your clichés. That’s not to say that personal and well-thought-out blurbs do not exist … just that you might never know for sure which blurb is done to keep the publisher happy and which blurb in an honest response to the book in question.
William Gaddis wrote in a letter to David Markson his views on just this subject and it is enlightening to read his words:

My feeling essentially is that a book really goes out on its own, for the human remains that wrote it to run along after it is suicidal since there’s clearly no separating them until the mortal partner drops. I don’t think ‘one decent blurb or two’ is going to alter Asher’s promotion at all, I don’t think lack of them is going to deter it; and the whole God damned area is to me like trying to make magic that will shape a course already implicit and then, if the course takes the feared-for direction, blaming the ex post facto magic, or the lack of it. I’ve never had my name on anybody else’s book jack or ad that I know of, I honestly do not think it would help sell a copy, it reeks a bit of self-advertisement though perhaps out of a deep mistrust for human motives or rather of them and the abyss between them and their expression this is merely an extreme inverted vanity on my part. Because on the other hand I do admire the generosity of people of stature like, say, Robert Graves, Norman Mailer, TS Eliot writing jacket blurbs for Faber, all of these people quite open-handed. I don’t know. I think of a boy I had at Univ of Connecticut working on a novel which I greatly encouraged, think publishable & have tried to help him place, he’s someone who’s never published and I hope to see have a chance, when/if his boo is published, what. I don’t know.
I have heard of authors that use a blurb on another author’s work for self-advertizement; I’ve even heard of authors anonymously posting self-congratulations and praise for their own works on the internet. Makes you wonder when you’re reading some of those unsolicited reviews: maybe they’re just a part of the fiction?