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As Google Blog Search updates much faster than the general index, these results can be found only using a blog search engine. At least for now. Then they'll become a part of Google's main index.
Google offers a lot of OneBox flavors, that enhance the search results by adding results from specialized databases or by providing a direct answer to your query.
Homework:
1. Do you pay attention to OneBox results?
2. If you can't find a good result for your query, do you try specialized search engines (blog search, Google Books etc.)?
Update:
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