As expected, Google has entered the blog search game. Try it here.
Note that it is not truly a blog search function, but a feed search tool. If a blog publishes only excerpts, Google will only search the excerpt. Also, it does not search comments or other material on blog sites. And it only covers posts dating back to June of this year.
Basically, it is more of a feed monitoring tool than true blog search. As time goes by and more feeds have been indexed, it will become more of a real search tool because the amount of available data will exceed missing historical data. (More at the Google FAQ for Blog Search.)