Of course, a writer can write about writing.
But an author can and should blog about any and all of the themes, issues, and historical periods that crop up in their books.
You can write about social issues that you feel strongly about.
You can share and defend any controversial opinions you hold. If a debate kicks of people will be talking about Insert Name Here, the author.
You can and should announce book signing tours, release dates, and awards.
You can even clarify some of the background mythology to your series (if people are showing an interest).
There is a lot that authors can blog about.