A puzzling reference question

A friend asks: Do you know if there is a way to search for blogs by subject, sort of like
yahoo? Like say I wanted to find blogs that focused on bonsai trees or
civil war poetry.

I played around with Google for a while, and came across a few places that claimed to be "directories" of weblogs, but most of them appeared to be woefully inadequate. Librarians, as one might expect, have done a reasonably good job of compiling on-the-fly directories of weblogs dealing with their own particular interests, but is there any good overall "finding aid" for the blogosphere?

Any ideas?

A corollary to yesterday's comments on weblogs: the widespread freedom to publish means that the resulting outflood of writing is nearly impossible to categorize or catalog in a timely fashion. Widely known but bears restating in this context.