The more niche oriented the content, the better your blog is going to work as a marketing tool. For lawyers and professional services business people, the niche for the blog can even be more focused than your area of practice.
What’s wrong with a corporate securities lawyer blogging about just one way corporate securities laws may be violated? Nothing. The blog will be more valuable to subscribers because of its focus. The lawyer will still describe in the promotional section of their blog the scope of their practice as covering corporate securities work beyond just that one type of violation.