Q&A for How to Make Tumblr Private

You can send the password through the "Ask" box on the invitee's profile if the person has questions enabled. The person can respond to the ask privately. You can also send the password via email. Never share the password in a public comment, not even for a few seconds or minutes. Anyone who is quick enough can get the information and possibly use it to harm or humiliate.

In blog settings, you can make your followers list private or public by toggle.

You cannot. Private blogs do not appear in search results at all, meaning they cannot be found unless you know the password.

A lot of things could be posted in a secret blog. For example, private info like passwords or things like sexually embarrassing pictures could be stored in a blog for only the owner to see. Also, it's common for a private blog to be used as a secret, online diary, and much like physical diaries, the owner may include confessions, secrets, and other thoughts that might hurt someone's feelings or get them in trouble, or just feel too embarrassing for the whole world to know about. They might cite personal stories that give away details like what school they go to, the names of their family members and classmates and teachers, and those stories might even be sensitive or hurtful.

Whether it reveals private information or not. In a public anon blog: Controversial opinions on celebrities, political topics, NSFW fanfiction, embarrassing thoughts that are general and don't reveal anything personal. In a private blog: Embarrassing content which give the uploader away, unique secrets or confessions about non-notable people (including the blog's owner), or secret conversations only the owner and a few close friends are open to (which may even be mean-spirited or illegal in the worst cases).

The pictures and stories are too unique even if certain information is censored or removed. The combination of certain details is recognizable to whoever is in the story or picture. Especially for pictures, the backgrounds as well as clothes and a combination of other things can be extremely identifying, not to strangers, but to the uploader's family and acquaintances if they accidentally see it. Also, the story on its own might not be identifying, but combined with other stories, their friends will find out it's them, even if no personal information is there at all, and it's easier to vent all those stories in one private blog.

A confession or secret would be unique if the combination of details is specific enough that anyone who knows the blog's owner will find out it's them. Even if no personal information is revealed, the owner may still get in trouble over it, especially if the secret involves people the owner knows or illegal activity. An online diary or secret chat would reveal private info since it talks about real-life events the owner and their peers are involved in, and they may reveal where they live or other specific details about themselves and their acquaintances which could get them doxxed or in trouble or embarrassed if they see it.

Personal diary entries or very specific, unique confessions that give away their identity to those who know them. In a diary blog, the combination of stories, even if names and certain other details are changed or removed, is identifying to people who know the blog's owner. They may not want to just censor personal info for their convenience, instead making it all private and open for only themselves to see. Secret conversations between a few people that are very identifying, no matter how much is censored or changed. Also, it may be hard to talk about it if too many code words are used.