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 Howard Krosnick, 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.
If you’ve written a how-to book, the best blurbs will come from people who have actually been helped by it. A good way to find “amateur” blurbers who might write sincere comments about actually benefiting from your book is to observe online communities that are concerned with your subject. If you find articulate people with problems your book solves, offer to send them free advance copies (even PDFs if bound copies are not yet available) in exchange for their comments. You can say that you’d like to know if the book was helpful and how it can be improved. Mention that you might like to quote their comments, but don’t guarantee it.
Short blurbs are usually better than long blurbs. Humorous blurbs (if appropriate) are often better than serious blurbs.
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.