I see that several more blog search engines have launched over the last few days, so I just did a quick survey of people at the coffee shop and over IM asking, "have you ever used a blog search engine to search for something?"
The answer I got from every single person? "No."
I'm not exactly in a little coffee house in the middle of nowhere either. This place should be filled with people right at the core of who should be using blog search engines.
I think that the real problem is that no one ever says "I really want to search for something, but I only want results about it from blogs." It's just not a compelling vertical.