When I am searching for library specific blogs I general use Google Blog Search http://blogsearch.google.com It helps but I still have to wade through a bunch of junk to find the types of blogs that I am searching for.
Liszen makes it easier to find library based blogs. You just type in the search term and it will only look at a subset of library specific blogs. What is interesting about Liszen is that it is built using Google's search engine architecture.
To simplify, users get to make your own search engine with the power of Google behind you. So if you wanted to create a search engine that looked for natural science web sites you can construct it with help from Google.
I don't want to mislead you. You need a certain level of technical skill to pull this off. For more information on how to get started visit http://www.google.com/coop
In the meantime I'm diving into http://www.liszen.com