"I will say what I think," I warn the seeker of said blurb.
"That's exactly what I want," the author, editor, or publisher will always say. "I want your honest reaction to (Insert Title)."
As a result, the friends I've lost and the editors and publishers that have posted armed guards to keep me and my mail out of their offices until the next Big Bang have become legion.
I've tried begging off, pleading ill health (usually true), the demands of my current opus (always true), but these ploys have the same effect. Depletion of friends, unpleasant responses from editors, emotional wreckage in my office that, frankly, interferes with my own writing.
None of which is to be taken as judgments on a particular author's writing. Stuff that I think is crap (defined as stories that don't appeal to me) get six-figure advances and win awards every year. Not having a Barry B. Longyear blurb on the cover doesn't seem to injure them at all. Of the few positive blurbs I have written on request, I have yet to have anyone say to me, "Well, your blurb on Alien Hookers Gone Wild certainly made up my mind. I just had to read that book."
One fellow who asked for a blurb had a somewhat snide response to what I wrote. Because I limited myself to only the positive things I could say about the book, the blurb came across as rather tepid. The fellow sent me a snail mail letter pissing and moaning about all of the really great things I could have written in my blurb, then, in a sarcastic postscript, he wrote, "Perhaps you can send me some writing tips." He even included a self addressed stamped envelope.
I busted off a few pencil points, put them in the envelope, sealed it, and mailed it back to him. Mission accomplished.