I started blogging around 2003, and back then, it was literally weblogging, i.e., posting updates in reverse-chronological order. It was useful for certain industries, like development, wherein such updates gave value to the reader by providing a glimpse into how a product or technology was developed, as an example.
During that time, the prevalent modes of interaction were usually forums and email groups. Blogs served as a sort of time machine that gave a time-based view of how something was developed over time.
Then it evolved into a web log of one's daily activities or thoughts. Sort of a precursor to our twittering or Facebook posting today. In many cases, blogging had been very short and informal. People did not mind grammatical or style issues.
So, it could go by several definitions. Is it the format (reverse chrono), the intent (regular updates), or style (free flowing)?
At some point, however, blogging started mimicking op-eds and news reports, complete with style, sourcing, and the known personalities writing. Perhaps what we're doing is a mix of all these.