Should you decide to switch it off, anyone visiting your blog at its URL will get a 404 error. Of course, your followers will still be able to see your stuff in their dashboards. "Just a simple layer of privacy to let you better control who gets to see your stuff and who doesn’t," the company said in a blog post. You can find the new toggle switch in the blog settings of your web dashboard.
This comes a few months after Tumblr introduced a feature dubbed Block that, as the name suggests, lets you prevent users from following/messaging you, like/reblog/reply to your posts, as well as see your posts/blog in search results.
Source
