For years Blogger's search of past posts worked well. Lately I've found it misses strings in some older posts, even strings that appear in post titles. So there's some unannounced limit been placed on search, possibly for performance reasons.
I hope Google will fix this, though it's a bad sign that they didn't bother to announce it. In the meantime I'm looking into what I can do with Blogger's export to "Atom Export Format". On first pass, it's not too promising. Gordon's Tech produced a 14MB XML file, including comments but excluding images (Text Wrangler opened it without blinking).
What I want is an HTML archive I can view locally and let Spotlight index. The sort of repository I used to create by running a robot against blogger.com. Maybe I need to try that again.