When you make your work available to the general public on a blog or a website you are publishing it. And while it's no longer eligible for the pristine title of “unpublished," it does have many of the same copyright protections that come with print publication. (See the Friends of Active Copyright Education for a general overview of copyright as it applies to the Internet.)
But blog posts don't always end on the web. Some blogs have gone on to make their way into print. Baghdad Burning is a book version of a blog written by an anonymous Iraqi woman calling herself Riverbend, who lived in Baghdad and offered eyewitness accounts of the realities of the war and occupation. In fact, there's even a word for books that come from blog content: blook. Not all blogs lend themselves well to books. In fact, many do not. For those that do, the process from blog to book isn't static. It takes into account what can be learned from the process of blogging and the interaction of the community that reads the blog. So, what ends up on the printed page isn't exactly the same as what appeared on the blog.Publishing individual blog posts in print publications as essays or articles is certainly possible, but it's rare. Print publications often prefer to give their readers something new, which is why so many of them require the work to be unpublished. Target the publications that are willing to look at previously published work, and before you submit, ask yourself: Why might this publication want to reprint something already available on my blog? If you can't come up with a compelling reason for this, you might be better off pitching something new.