Dave Winer (of Userland) has written a piece on What makes a weblog a weblog?
This has a bibliography that is short but good on the subject.
In my view a blog is chronological, it has a personal and informal voice, it points typically to other Web phenomena, and it is potentially never ending. One could look at it following Bahktin in terms of its chronotope:
Space: On the Web with links to other Web blogs and sites. It comes from a personal space – it is someone’s blog who resides somewhere and has a certain perspective.
Time: Presents itself as episodic – a personal journal organized chonologically around a calendar.