If you Google how to make a Tumblr blog private, you’ll be shown many search results of doing exactly that, and it looks quite simple. They all show the same thing: Head to your blog setting, check the “Password protect this blog” and boom, you’re done.

But it’s not that simple, and you can trawl through your settings until your mouse hand is blue in the face, you won’t find it.

After much fighting and arguing with Google, hidden deep in the echelons of Tumblr’s staff posts and help, I eventually found what I was looking for, and it’s quite simple.

There are two types of Tumblr blogs. The first one you created, your primary blog, is the one which uses your primary username. Then, subsequently, you can create many secondary blogs which piggy-back off your primary blog.

YOU CAN ONLY MAKE A SECONDARY BLOG PRIVATE, NOT A PRIMARY BLOG. So stop looking, you can’t.

To make a secondary blog private:

  • Click “Settings” under the Account menu at the top of the Dashboard.
  • Click your secondary blog name in the menu on the right side.
  • At the bottom of your blog settings, enable “Password protect this blog.”
  • Enter your desired password. You will see a green “saved” indicator to let you know you’ve password-protected the blog successfully.

You can read more info here: https://www.tumblr.com/docs/en/blog_management