An update to Fetch and Render shows how these blocked resources matter. When you request a URL be fetched and rendered, it now shows screenshots rendered both as Googlebot and as a typical user. This makes it easier to recognize the issues that significantly influence why your pages are seen differently by Googlebot.
Webmaster Tools attempts to show you only the hosts that you might have influence over, so at the moment, we won't show hosts that are used by many different sites (such as popular analytics services). Because it can be time-consuming (usually not for technical reasons!) to update all robots.txt files, we recommend starting with the resources that make the most important visual difference when blocked. Our Help Center article has more information on the steps involved.
We hope this new feature makes it easier for you to spot and then unblock resources used by your website! Should you have any questions, feel free to drop by our webmaster help forums.