(It's a silly question, I know.)
In the United States, a copyright expires 70 years after the death of the author (source). With the relative infancy of the Internet, we're still several decades away from the day when practical debate is required over whether royalties are still due to the descendants of bloggers born more than a century before them.
A couple of issues drove the economics of public domain books in our history: the monopoly pricing of original content and the transactions costs of tracking down the descendants. How does these things change in the Internet age, especially for content like blogs?
Milking the Monopoly
If only I can publish copies of "Moby Dick," I can charge monopolistic prices to maximize my profits; this is assuming that people cannot easily steal the work through photocopying, downloading illegal Kindle copies, or whatever other method (I'll drop this assumption later). Some people will choose not to buy "Moby Dick" at my exorbitant prices, but I will make much more money from the people who do.
If everyone can publish copies of "Moby Dick," price falls to marginal cost, or the cost of physically making the book. If I can print and distribute a copy for $5 and choose to charge $6, someone else can enter the market and undercut my price to $5.50. And on and on until the price approaches $5.
To undermine my previous point a bit, I've noticed that several James Patterson books on Amazon are selling for roughly $10 a pop for new paperbacks, as are most editions of "Moby Dick" (though there are a few exceptions, around $5 a copy). So one has to wonder how much of a markup publishers can get away with; after all, no book is truly a monopoly, as it must compete against other books. Yet the copyright holder's markup is apparent in the Kindle market, as most eBooks are $9.99 but all the classics are free (as opposed to traditional books, each copy requires only a trivial cost to distribute).
Blogs do not face this same marginal cost dynamic. Assuming that Web hosting costs are negligible or free, each "copy" of a blog post that is distributed costs the writer nothing (after he has taken the time to write it) and the reader nothing. Each page view has a slightly positive expected profit, as some of the readers will click ads and generate revenue, so there's theoretically still some money for the descendants to collect. But because neither the publisher nor the reader has to pay to distribute or access the content, a public domain law seems a moot point: there is no monopolistic price to combat. It seems sensible to let the descendants continue to collect the advertising revenue (keeping in mind the caveats I present in the next section). What are the alternatives? We could give the ad revenue to charity or put it toward the national debt. Or the ads could be removed, but that would require a lot of legwork (how hard will it be to modify Web sites made 80 years earlier?) and make the reader's experience only marginally better.
Some online content, such as articles in scholarly journals, are behind paywalls and thus do not operate the same way. The classic public domain model used for books seems most appropriate here: under the spirit of our current laws, someone shouldn't be able to keep content behind a gate forever. Of course, as digital "theft" becomes easier and easier, perhaps the content-behind-the-wall method will cease to be viable anyway.
Much blog content is news-orientated, so it's doubtful that most of it will be useful, or popular enough to generate substantial ad revenue, decades from now. How many posts are being written every day about the new iPad, and how many of them are still going to be read regularly 10 years from now?
Tracking Down the Descendants
Throughout history, keeping track of descendants generation after generation has been difficult. Additionally, as time goes on, a common ancestor can have a multitude of descendants: imagine if you have two children, who each have two children, who each have two children, etc. Both of these reasons are also good economic justifications for our current system. Most people would agree that there shouldn't be several thousand people getting royalty checks because they are distant descendants of Socrates, and even if they were entitled to it, the administrative costs of keeping up with so many heirs would probably eat up most of the purse anyway.
The Internet solves the former problem but not the later. It's easy to imagine a Web site where authors could designate heirs, who in turn could designate future heirs, and so forth. But we have not solved the second problem: should the revenue from a centuries-old blog be split among hundreds of people so extremely far removed from the original author? If not, what else should we do with the money?
Of course, it's possible that technology and our world will change so dramatically in the coming decades that none of these issues will be applicable, in ways we wouldn't be able to understand from our current vantage point.