Basically, there are two ways to searching all the blogs in your Google Reader. One is to use Google custom search engine, and the other is to use Google gear and Greasemonkey script.
The details about the latter option is described in Raúl's blog. The offline function of Google Reader works for me, but the Greasemonkey search script halts everytime I try to start the offline search. I cannot figure out what is the problem.
What I currently use is the first option: create a custom search engine that has all the links of subscribed blogs in its site list. The details is described on Google Operating System blog. The search function really helps when you want to testity some 'Déjà vu' of your reading.