You don’t have to be an expert to write good copy for a blog, but it probably helps. Experts seem to be taken more seriously and read more. Have you seen how many thousands of followers some of these expert bloggers have?
A journalist once told me that good research and a tight deadline enabled him to write expert articles. After 9 months of writing copy on a particular subject, he felt confident enough to write a book on it. 9 months to become an expert
Of course, you still have to write your copy and here the advice is constant from many sources. You just sit there until you have written it.