Unlike fiction readers, people who buy non-fiction books are looking for information. They may be curious about facts or have a problem they want to fix, but they're all looking for factual answers to some question. Your book blurb's job is to convince them that, not only do you have the answers in your book, you know what you're talking about and you can share the information in an easily understandable manner. That doesn't necessarily mean simple, just that readers can figure out where you're going.
Split your blurb into three sections, and write it in third person. You're striving for a non-biased voice that's simply giving information.
The first paragraph of your blurb is where you tell the reader the benefits of reading your non-fiction book. What's inside? What need will it fullfill, or what question will it answer? Tease an insider fact or two, to tempt them into reading further. Give the reader some solid information about the contents of your book, enough so that they can tell how thoroughly you've covered the topic.
The second paragraph is for endorsements and testimonials. This is something you should start working on long before you've finished your books. Begin with your relationships, both online and offline. Look for others in your field who are willing to read your book in advance and give it a glowing one-line review. Mentors are good for this, as are known names in your niche subject. Make sure they're people who know you or your writing well, so they can give honest feedback on your work. Include two or three testimonials in this part.
This last section is where you show readers your credentials. Establish yourself as an authority on this subject. This doesn't necessarily mean academic degrees. If you're writing a gardening book, having grown the same type of plants for six years in a row is a more valid experience than learning the subject through books. Use this section to let the reader get to know you a little bit. Personalize it with your experiences in the subject, so they feel as if they're experiencing it right along with you, as much as possible.
All three sections of your blurb work together here to convince new readers that your book is the one to buy. It's your first and best form of book marketing. Tell readers what's in the book, who agrees that they should buy it, and why you're the best person to write it. These three topics combined create a compelling statement that will draw readers in.
Keep the Faith and May the Force be with You!