Yes. Your content is definitely private and secure.
As your book is produced, any individual who might see or otherwise interact with the content as part of the production process is bound by employee/contractor confidentiality agreements. We have never encountered any problems on this front, and we intend to keep it that way.
Once your book is produced, you control the privacy settings. The default is that the book is "private," meaning only you can view or purchase the book. You can put your book in Blurb’s public bookstore where anyone can purchase it, set it up so it can be seen by invite only, share your book on social media, or keep it completely private. The degree of visibility is up to you.