1. Serverside - block Search Engines
Create a robots.txt file in your root directory and add this text to it:
Theoretically, This should block all search engines (the ones that honour robots.txt).
2. Know how Search Engines are crawling your site and control your site's visibility.
Since you want it to be extremely private, I will recommend that you sign up with google webmaster tools and make sure that your blog is not being indexed by google.
GWT is a bit late to reflect realtime search results so you can also do a search for
site:yoursite.com to keep an eye on realtime indexing.
If you see something indexed that you do not want, you can remove it using this link: https://www.google.com/webmasters/tools/removals
You can follow the same procedure for bing, Yahoo and any other search engines as they provide similar tools to control your site's visibility.
3. Serverside - Password protect your blog
If the content being shared is of extreme sensitive nature, I would recommend password protecting the site (and sharing password when you share the link). If you're using wordpress, here's a good plugin that will help you achieve that.