I had a friend, and I had a secret. I told my friend my secret - and now, I have neither a friend, or a secret.
Occasionally we see signs of naivete, in Blogger Help Forum: Something Is Broken.
I have a private blog! Why do I see my blog listed in Google?
I distinctly set my blog to be invisible! Why is my blog being indexed?
Not all blog owners know that neither the "disallow" statement, in "Robots.Txt" - nor the "noindex, nofollow" directives, in our blogs - are mandatory.
If you publish content on the web, chances are it is - or will eventually be - indexed somewhere, by some search engine.
Obedient search engines won't index content, when instructed.
Obedient search engine robots will observe the Google privacy directives, such as "noindex, nofollow" directives in HTML code, and the instructions in the "robots.txt" file. Not all robots are obedient, however.
There are many search engines besides Google - and not all search engines observe privacy directives.
The various robotic processes, which scan for abusive content, scan all blogs and websites out of necessity. Some archiving robots archive everything - not just content that's provided openly.
Many search engines share indexed content - obedient, or not.
Many search engines share data with other search engines, and / or retrieve data from other search engines. Even Google will index some sites, indirectly, that are not intended to be indexed.
Ownership and privacy laws vary - from country to country, and between Internet services. What you consider private (personal data), if you live in Germany, may be treated as common knowledge, by someone in Russia, or the USA. And if you report someone publishing your private data to Blogger / Google, you may get different treatment than from WordPress, or from an independent hosting service in China.
If you publish content, expect that it will be indexed.
If you want to keep a secret, do not publish your secret on the Internet. Regardless of whether you publish it to a blog with designated readers, using the Blogger dashboard Privacy settings, or even using a custom "robots.txt" file, your secret may be visible on any search engine - even on Google.
If you think the Internet is your friend, try publishing a secret there. You will learn the truth, eventually. And hiding your secret, after it gets out, will not be an easy task.