Short answer: October 13, 1994.
If you look at the chronology of DaveNet, you'll see it wasn't the first piece, it was the third. But it was the first I wrote after realizing that I could publish my ideas on the Internet, without going through a publisher. The two pieces before that were more like notices or ads. This one was meant to stimulate thinking among people who were reading. And boy did it ever stimulate thinking! What came back was a virtual avalanche of ideas, from some very smart people. I published the most interesting response (to me at least), back out through the same channel. And then I did it again, and again, and on and on. I'm still doing it almost 17 years later.
I'm sure people will dispute that date, it's the nature of blogging to dispute things. But I was there, and you probably weren't. And from that point on, it was a sequence of steps that eventually led to all the things we consider part of blogging today.
Some people who have studied this stuff minimize the ideation process, the evolution of blogging from virtual nothingness to a literary practice (at its best) that's capable of delivering great insight and feeling. If you doubt me, look at Nobel laureate Paul Krugman's dispatch of former Fed chairman Alan Greenspan. That's a bit of on-the-record writing that belongs in a history book. And it was written in the last 24 hours. The freshness and directness are, to me, the best of blogging.
Excerpt: "He's no longer the Man Who Knows; he's the man who presided over an economy careening to the worst economic crisis since the Great Depression -- and who saw no evil, heard no evil, refused to do anything about subprime, insisted that derivatives made the financial system more stable, denied not only that there was a national housing bubble but that such a bubble was even possible."
Designing software is hard work. The things that seem obvious after-the-fact were anything but obvious before. The best things, the most useful innovations, are the ones that seem so obvious later that they melt into the fabric of reality and become invisible. I am proud to say that many of the things that came from the evolution of blogging have turned out that way. Like all art, software design strives for suspension of disbelief. In our world, that effect is felt by the tools becoming so invisible that you can focus all your attention on your work. It's creativity for the sake of other creativity.
There are other milestones for sure. I remember the frustration of the flaming mail list for the 24 Hours project and realizing I could side-step the flamage by publishing a reverse-chronologic list of links to pieces of the project. In other words, a link-blog.
After that, came Frontier News, a link-blog for a scripting community. Which then led to Scripting News, and that blog is still running today. But the beginning, the first blog post authored by yours truly, was the open letter to Jim Cannavino on October 13, 1994.
PS: Why I don't do interviews, in a blog post, of course.