"It's a blog not a journal." That blurb was posted by an anonymous commenter on a newspaper blog. It's an interesting opinion. Unfortunately, the rest of the internet does not agree.
No better proof than the dictionary. Dictionary.com defines blog as "a journal written on-line and accessible to users of the internet." Merriam-Webster declares blogs are "an online personal journal." Wiktionary states that blogs take "the form of an online journal."
If you do not consider dictionaries as definitive substantiation, take Blogger's description of their own service: "Blogger is a free blog publishing tool from Google for easily sharing your thoughts with the world." The creators of WordPress give their users the following bit of encouragement: "Express yourself."
Everywhere you look, sources tend to coincide. The Online Journalism Review calls blogs a "series of entries to an online journal." New Zealand museums tell us that "Blogs operate much like an on-line daily or weekly journal." FoodInfoQuest propose that blogs are "typically a hybrid of a personal diary and a commentary." And the Parliament of Victoria ruled that blogs are a "web log or online diary."
Still not overwhelming evidence?
Let's check the supreme and absolute source of all knowledge... wikipedia. "Many blogs provide commentary or news on a particular subject; others function as more personal online diaries."
Blogs are a place where anyone can voice their opinion. It's also a place where we can point out when the opinions of others are wrong. In the case of the nameless person that thinks blogs are not journals, the vast reservoirs of the web dispute your claim. You are incorrect: blogs are journals.