Tumblr is a big conversation. Or, more accurately, a whole lot of different and overlapping conversations. It’s different from platforms like Blogger or Wordpress in that regard.
Following someone’s personal Tumblr usually means you’re trying to join their conversation. It’s more personal and interactive than subscribing to an RSS feed or following someone on Blogger or Wordpress. Doing that when they don’t want you to is invasive.
It can be confusing to sort out what’s what sometimes. Reblogging something enters a conversation, and it can bring a lot of people into the conversation who the original poster hadn’t intended to talk to. Sometimes this is ok, and sometimes it isn’t.
Sometimes reblogging something can attract a pile of angry people to attack someone. Sometimes it can attract a pile of angry people who don’t understand what the OP meant and don’t want to.
When someone appears not to want to talk to you, it’s usually better to stop reblogging their posts and start your own posts instead.
When you don’t want to talk to someone, it’s a good idea to block them. That won’t stop them from reading what you say, but it will mean they can’t message you and you won’t see them. That can make it possible not to get sucked in.
All of this is public in some sense – but it’s not public in the sense that a newspaper is public. It’s public in the sense that having a conversation while walking down the street is public. In that, it’s not ok to follow people around or join their conversation if not invited to do so. But people don’t have a resonable expectation of secrecy, either.
Some Tumblrs are really public. This one, for instance. I’m talking to anyone who wants to listen. But when people aren’t, it’s best to leave them alone.