For the past few years Apple and Adobe have been at odds with each other over the use of Adobe's Flash Player to display content on the web.
Every few months, the battle heats up. Right now there is a security issue with an older version of the Flash Player and Apple has taken steps to prevent it from loading in Safari. You might notice this when you go to a web page and see this image:
When you see this image instead of a video - your flash player is out of date.
Adobe has posted a Knowledgebase article with more details, but the short version of the instructions are: download and run the Flash Uninstaller from this link.
Once Flash is removed, you can download and install the most current version from the Adobe website. Click on the Download Now button and run the installer.
Once Flash is up to date, the Blocked Plug-in messages will go away.