I'm just starting to use the search box gadget, but
Google Custom search coverage of my blogs varies from very good to fair. It all depends on how Google feels about my blogs on a particular day. Sometimes it's keen, sometimes not so interested. When it's keen the material is well indexed, when I'm out of favor the indexing is shallow. It's never comprehensive however. (Google's variable affections reminds me of my 7yo daughter, but that's another story.)
Google Blog search has extremely poor coverage of my blogs. It seems to largely index the popular blogs.
The search function that's displayed in Blogger's NavBar is sometimes better than Google Custom Search, but it's shallower -- biased to newer stuff.
Finally there's the search function that's part of the Blogger editing environment. I thought that one was really comprehensive. I'd never known it to fail -- until today.
Google Custom Search found one of my very old (2003) posts in Gordon's Notes, but the Blogger editing search couldn't find it. I did some probing and it looks like I'm indexed from 2004 on, but it fails in 2003.
So from Google alone I count five different ways to search my Blogger posts, but none of them is perfect. The Blogger editor search is most comprehensive, but the Google Custom Search will sometimes turn up posts it misses.
Of course if this material were all on my hard drive Spotlight would index it all. The Cloud is not always your friend.