Tumblr’s added some ruidimentary search operators, one of which lets you search by blog, and it actually turns up posts, unlike the search bar on a blog itself. It recognises quotation marks now, too.
You just have to make sure you’re searching on the actual “search” page, it still tries to take me to the “tagged” page and I have to manually change the URL.
BUT, you can actually find a post on someone’s blog now. Just don’t start from the blog you want to search
