Blog posts have an option that controls who is allowed to see the post. There are up to three choices, in increasing order of visibility. The first choice is different for a personal blog compared to one that is on a course.
On a personal blog: Visible only to the blog owner (private) – nobody* else can see this post.
On a course blog: Visible to participants on this course – to view the post you must have been granted access to the blog, usually by being enrolled on the course that contains it.
This option exists on the whole blog as well as on individual posts. If the option is set on the whole blog, that becomes a maximum. For example, if the whole blog is set to the first level, you cannot change the level of an individual post at all.
* System administrators have access to all data stored on this particular computer system, including blog posts that are marked 'private'.