The point of having a blog is to build your platform—to establish a presence in the marketplace. Therefore the blog should be about you and the topic that you write about, rather than just a random selection of observations.
Are you concerned that if you set up a blog you’ll give too much away for free? Dustin Wax addressed this issue on the Freelance Switch blog, and suggested that writers' blogs should add value to their writing, not replace it. He suggested posts on topics such as:
• advice
• behind-the-scenes peeks at your topic
• reviews of relevant books you’re reading
• discussion of news events that impact your readers
As Dustin points out, building a platform is a long-term effort and it benefits from having a presence in a variety of ways.
(Of course a blog may not be your only outlet. My others include the websites www.timetowrite.com, www.yourwritingcoach.com, and www.focusquick.com.)