To my knowledge, there's no good way to find blogs. Blogsearch are filled with spam and are more post search than blog search (i.e: the one result returned looks good but the rest of the blog isn't relevant), blog directories are plenty full but lack quality. It's easy to find a few blogs, say 50, but it's very hard to find all of them for a particular niche (and an organization that want to be serious listening to conversations has to do that on more than just a few). It's not a one click affair, it's a process. I've assembled huge list of blogs (1500 blogs on personal finance, 1000 blogs about blogging) and none of the tools mentioned above were good enough. So we built our own mechanism which is a mix of human sweat and machine algorithm. Our clients are happy we did that for them as now they can follow and engage with head/magic middle and long tail blogs whenever they are relevant to what the blogs is posting about.

Another way to identify top blogs is to just to pay me to do it for you:)