I'm a big believer in casting a broad net when you search, at least at first. But it can be hard to sift through the zillions of search results that you may get from the mainstream blog search engines. There's just SO much. So specialty search engines, such as BlawgSearch, that searches only the legal blogs, can be handy.
If you're looking for stuff that's strictly library related, you have another option, Libworm. LibWorm searches over 1500 library-related rss feeds. You can also browse a list of subjects, or feed categories and view a Libworm tag cloud. You can even create an account and using MyLibWorm as a sort of RSS reader, albeit a very limited one.
LibWorm is currently in beta, so there's hope that there could be improvment in the relevancy ranking. Many of the categories, such as the Law Libraries category, simply don't return useful results. Maybe I'm missing something, but I'm also unable to find a list of the blogs that are searched. A blog directory, organized by topic, might also be a handy addition.
I would agree that the content is a little sparse, especially for the law library realm. It would have been nice if all of Bonnie's List could have been included.
On the positive, I've noticed the updates to the collection via RSS are very fast, and in turn, the RSS feeds delivered are in my aggregator faster than most of these niche search products. FWIW. :-)