A Steemit Blog is where user posts appear on Steemit. Blog posts are arranged in a chronological order.
Blog is the second menu item in user account menu. You can get to your account menu by clicking on the avatar icon in the top-right corner of a Steemit.com page.
The link "Posts" on user profile page refers to both blog entries and comments.
The modern blog evolved from the online diary (the word "blog" itself is a truncation of "web log") where people would keep a running account of the events in their personal lives.