Often, especially for a new author with a new book, it's just not possible to get the attention of a a superstar or an expert who will add authority to yours.
That doesn't mean your book has to be blurbless.
There's nothing wrong with asking for and printing blurbs from friends and family, if it's appropriate to your book. Later on, If Oprah or another celeb falls in love with your words, you can revise the cover to incorporate the new comments.
My first self-published book I Only Flunk My Brightest Students -- stories from school and real life, deals with my life. So it made perfect sense to use blurbs from people who know me, rather than some distant Nobel Prize winner.
The book is funny. Identifying the source of my front cover blurb as "author's classmate since first grade" is almost a parody of the traditional stuffy IDs ("professor of Indo-Eurasion folk medicine at the University of Guatemala), and reinforces the mood of the book.
Stories I'd Tell My Children (but maybe not until they're adults) is an updated replacement for the flunk book. It has a fantastic cover blurb which says, "This book is so funny that I nearly peed in my pants. My girlfriend didn't think it was funny, so I got a new girlfriend."
The blurber, Nicholas Santiago, is someone I know through business. His words are sufficient. I see no need to explain who he is, and I doubt that Oprah could have written a better recommendation. I received "five stars" and some nice words from the Midwest Book Review -- but those words are not as funny as Nick's words.
There's nothing wrong with your acting as a writing coach for your blurbers. You can even write a complete blurb and ask someone to "adopt" it.
Request blurbs as long in advance as possible -- as soon as you have a draft of your book that is good enough to show. The book does not have to be complete. You can probably get by with an introduction, a table of contents, and a few chapters sent as a PDF. If you want a blurb from someone famous, it’s probably better to send an ARC than a PDF.
Incorporate good “early” blurbs into your back cover and first page as soon as possible. If other blurbers read them, they may be more likely to write similarly positive comments.