To those who wish to not display their Tumblr blog, there is a toggle switch for you!
The “Show this blog on the web” toggle will hide your blog when accessed through its URL. They will see a 404 error web page.
Even when switched off, your followers will still be able to see your posts on their dashboard. They can still like or reblog them as they please. The new toggle switch is in the blog settings of your web dashboard.
Source: Tumblr