You can search for posts within a specific blog from your dashboard. Click a blog’s name or avatar to open it up, then click the magnifying glass at the top of the blog. Now type. Type like you mean it. Get your results.
(And as we've mentioned, you can filter and sort those results by clicking the abacus-looking thing next to the search field.)
You can also search a blog directly from the blog’s URL, as long as their theme includes a search field. But why take the risk? Just use your dash.