I think it's important to remember that Joel did it the other way around-- the blog preceded the business.

Joel built up an audience, and simultaneously built a product that would appeal to this audience. But even that makes it sound too calculated, as Fog Creek's first product wasn't really aimed at Joel's readership at all.

Joel wrote about something he was passionate about, and wrote well. He also made a product that resonated with decent chunk of his readership.

Here's a good test: would people who have no interest whatsoever in your product be interested in your blog? If so, it's a personal blog. Cool. (Just remember that your readers are not necessarily your customers, and vice versa.) If not, it's a marketing tool. Which may also be cool, if that's what you're after.