I've noticed that the blurbs on book covers that are written by colleagues (are they called "endorsements"?) usually seem to be from writers from the same publisher. That makes a lot of sense, but what if your best friend is a novelist with another publishing house, she loves your book, and really wants to write a blurb for you? Will your publisher allow it? Will hers?
The quick answer: for sure. In fact, I currently have six blurbs for my book, and only one author is published with my house, and she's not even at my imprint. Of course, I got these blurbs on my own: I reached out via email to authors whom I thought might enjoy the book and with whom I might share an audience. I didn't know any of them beforehand, so I simply jotted down a (very kind and humble) introductory note and asked them if they'd be willing to take a peek. Most said yes. I made it clear that there was no expectation of a forthcoming blurb, and if they hated the book, then they should definitely not endorse it. I've been fortunate that most people accepted and that most have indeed had very nice things to say. (Though I'll admit to being heartbroken when Lolly Winston couldn't fit it in her schedule. Wah! I still love her though! And can't wait to read her new book!)
Remember that blurbing is in some ways a two-way street. Trust me, I was so, so, so, so flattered that these wonderful ladies not only took the time to read the ms, but also to write something so wonderful about the book, but they also get their names on the back of the cover, or on my website or on my blog or in the press kit or wherever. So it only serves the author or the publishing house well to open themselves up to outside authors.
My process of gathering blurbs was a bit unusual - most often, it's left up to the editor, the publicist and the agent, which is why you often DO see authors blurbed from the same house or imprint. But I just wanted to be a bit more involved, and I'm glad that I was.
This question of blurbing raises a whole other issue: do blurbs matter? I know that as a reader, I definitely take a look at them and and that they can tilt the scale if I'm debating a purchase. And if I've enjoyed a book, I'll often buy a blurber's book because I assume that we have similar taste. Diana Peterfreund blogged about this a few weeks ago: whether or not blurbs really have any influence on readers. They most definitely influence booksellers (i.e, if the press kit is brimming with top shelf blurbs, they'll likely be interested in buying more copies) and get your sales and press teams fired up, but to the average reader, do they really do much? I dunno.