Areas of focus are the values that completed projects fulfill. In the context of a professional blog, a project can range from writing a post to installing a plugin.
Areas of focus can be viewed on two levels. The "org chart" perspective deals with what roles need to be filled to produce a successful blog, like creative development (topic research and brainstorming), content production (the actual writing), marketing, SEO, design, monetization, and so on. What boxes need to be ticked to feel like your blog has a healthy momentum.
Then there's the area of focus that the blog itself fulfills in your life. For instance, if by "professional blog" you mean a blog that's meant to raise your industry profile and consulting fees, then "career development" might be your area of focus. If you mean being a "problogger," someone who earns a living through blogging, then the need to extricate yourself from the job market might make "financial independence" the area of focus, as opposed to creative expression, or some other nonprofessional slant.
For project planning -- at least on the content front -- I have a calendar category that only contains the title of the post planned for each day of the week (like Brent, I brainstorm my topics on Sunday). I like to see them laid out sequentially. Once I have the title topics, I drop these into MindManager (until recently, I was using CmapTools to do this), then start collecting details on each topic until it's matured enough to draft a post from. When it comes time to write the post, I switch from Map View to Outline view in MM, and draft point by point from the outline.