When you look at the synonyms for journal on Microsoft Word, it gives you several options, including paper, bulletin, and newsletter.
So, blogs are like a public journal because no matter how you write or what you write, each blog post is like a journal entry into the very impersonal world of the internet.
Some bloggers pour out their hearts and souls into their blogs, while others only share some details of their private/personal life. Whether you are blogging about intense personal details of your life, such as weight loss, marriage, and/or children, or you blog about general topics such as books, music, or current news, some part of you goes into your blog, which I believe therefore turns your blog into a journal.
Check out my previous A to Z Challenge blog posts:Blogging: G is for Giveaways
Blogging: H = Headline
Blogging: I is for Images