If you self host, here's one option that might work, particularly if you own multiple domains. Consider installing Google Custom Search. If you use the free install option, it's pretty easy. During the install, you have the option of listing the particular areas that you want to include in any potential search. This is the section that you should list all of your other blogs. Why? Because in order for Google to accurately provide good search results, every one of these blogs must be indexed regularly. Seems logical that unless Google wants to provide crummy results, it will go out of its way to spider and index any site you connect to the search function. I've done it on several sites, never had any of them de-indexed and all of them appear to have been fully indexed when I do a site-specific search on google (site:zzz dot com, search phrase)
Best of luck.