Brandon Wason, Mark Goodacre, and Jim West all kick around the definition of a biblioblog.
Wason/West: "...a weblog that focuses primarily on Biblical Literature, related fields, and occassionally contemporary events. It's purpose is to offer news, opinion, and conversation for those interested in the Biblical text. Biblioblogs occassionally refer to personal matter, but that is not the primary focus."
Goodacre (more simply): "...blogs which have a primary focus on academic Biblical Studies."
Jim West notes further that while some biblioblogs are "pure", focusing almost exclusively on biblical studies, most of them throw personal, political, and theological postings into the mix.
I'd say that probably at least two-thirds of a biblioblogger's postings should relate to the topic at hand (biblical studies), though forays into unrelated or quasi-related territory are nice too. Sub-topics of my blog include the theological world-view of Tolkien's Middle-Earth, and the field of evolution/evolutionary psychology. Sometimes sub-topics relate back to the main. For instance, I recently compared the way Paul uses the figure of Abraham with the way Tolkien uses the character of Sam. Next week I want to post on the phenomenon of lying and deception, as it applied specifically to people from the biblical world, and more generally to homo sapiens as a species, and what we can say about "lying" in general. (So we need to brace ourselves for strong doses of cynicism next week.)
Most are aware of Biblioblogs.com, run by Wason and West. They offer a good list of biblioblogs, though there may be more out there which deserve to be included.