Is it possible to search upon a specific web site or blog?

Not without giving you command line access to the application, unfortunately! One of the challenges in delivering this feature in a satisfying way is making sure we get everything from what you consider a "source." Until we can ensure that when a user enters "techcrunch.com" they get "mobile.techcrunch.com" and "techcrunch.fr" and content from other sources which are from that TLD, whereas when you search for "blogs.wired.com" you get "blogs.wired.com/science" and "blogs.wired.com/technology" and NOT everything from the "wired.com" TLD, we probably won't make it generally available. Look for this in the summer timeframe.