[This post assumes the definition of art that I outlined in Are Games Art? Please don't comment on this post if you didn't read the original post. If you disagree with my definition of art, comment on the original post, not this one.]
The question "Are Blogs Art?" has been asked surprisingly infrequently, probably because the answer is obviously "yes" to most people, I guess. If writing a book is art, and writing a short story or taking a picture is art, then surely blogging is art.
But consider a book of pictures by Monet. Each picture is art, surely, but is the book art? The collection of pictures?
When considering an "art form", you must take the result as a whole. You analyze the form beyond the mere individual parts that make up the whole.
A single short story is art. A book of short stories is art only if the collection conveys something in its own right. For what it's worth, copyright laws recognize that a particular arrangement of material can be art, independent of the materials themselves.
So for a blog to be art, we must look beyond the individual postings, which of course can be art. I think we should look beyond the design of the template, as well, which also is, or can be, art.
What is Art in an Art Form?
In a vast majority of art forms, the "piece" of art forms a coherent whole. A piece of music starts and ends, generally communicating a discrete theme or set of themes. These themes are interwoven throughout the piece, through repetition, buildup, or however. The same goes for books and paintings, and most other art forms.
When it comes to interactive performance art, however, such as games or participatory art, the art form may be one that involves an ongoing process, where people can walk in and out of the experience. As such, the art is in the mechanisms that facilitate the interaction.
These mechanisms don't have to guarantee an artistic experience for everyone who touches the art, nor the same experience for those touched by it. In fact, the themes and messages of the mechanics can evolve over time.
An Art Installation Example
Consider an art installation as follows: A room, where each person going into the room picks up a piece of paper and reads it out loud to the next person who walks in. The next person does the same with a different piece of paper. Assume an infinite supply of pieces of paper.
It has particular beginning, and no particular end. The themes on the pieces of paper might change all the time. Nevertheless, if properly controlled, the exhibit itself is art (by my definition), and not simply the items that are read out loud.
This "art", which is about meaning, connectedness, and many other themes, changes continuously, not only because each person will experience something very different, once as audience and once as reader. But also because as people go through the exhibit more than once, the experience changes. As people learn more about the exhibit and it rises or falls in popularity, the experience also changes.
When I talked about games, I also distinguished between the rules, pieces, design, and so on, and suggested that for games to be art, the art must come as a result of something "weighty" being conveyed by the actual experience of playing the game.
The Answer and an Example: Postsecret
So if we talk about blogs, the art from a blog must come from something artistic about the experience of the blog itself.
Is this possible? Sure, it's possible. Look at Postsecret.
The postcards sent in to Postsecret may or may not be works of art on their own. Each individual post on Postsecret, each of which contains a collection and arrangement of postcards, may or may not be art. But the overall experience of Postsecret, week after week, communicates something in its own right. Community, connectedness, loneliness, shame, whatever.
Postsecret calls itself "an ongoing community art project", and it is, much like a mural of individual drawings expresses more than the drawings do themselves.
The Next Question
So I guess the real question is: is your blog art?
First of all, it doesn't have to be. The vast majority of you who blog just because you want to blog, go ahead. Like keeping a diary, there is neither expectation nor necessity to create art while doing so (we can't all be Anais Nin).
For those of you who want to create art, you can do so by creating artistic posts: pictures, words, and so on, without your blog being art in its own right. That is also enough - more than enough. It's wonderful.
For those who want to create art out of the blog form itself, you have to ask yourself: what is your blog conveying? Is it tackling a theme? Are these themes repeated or conveyed by the blog itself, either through the arrangement of your posts or somehow through interaction with the readers?
I welcome examples of what you think are artistic blogs.
 Diary of a Website considered the way different blogs do different things while asking this question.
 Blogcritics talked about the embryonic art of blogs.
 Silflay Hraka considered the question in the first Carnival of the Vanities.