I saw a novice author asking a famous one if he’d read his novel in manuscript and write him a back-of-the-book-jacket blurb. I turned away, not wanting to see either of their faces when the inevitable "No" came. I’ve written blurbs and have always read the whole book before doing it. I've also been asked to read manuscripts and write blurbs for them when I was so overloaded with work I couldn’t possibly do it, and I even tried, but gave up after a few chapters and with regret told the author that I couldn’t write a blurb. But—silly me! How many books have you seen with blurbs from, for example, Stephen King? A lot! All those famous and prolific authors giving each other blurbs on the backs or even fronts of their books—they didn’t have time to read all those manuscripts and write blurbs for them either!
Do you know how that's done? A trick: The author of the manuscript writes the blurb and sends it for approval to the desired blurb writer. The blurb writer either signs his name to it or not. As long as the desired blurb writer is a friend or colleague it's easy. (Never ask a writer you don't have a reasonable personal connection to.) You might say the auto-blurb is not a genuine opinion then, but when you ask for blurbs you aren't asking for unbiased reviews. It’s a blurb: a marketing tool.