I opened blueprints prior to installing your plugin and my mate is still showing locked items that I have already opened for myself.  Does this plugin only work on blueprints that are opened after it's installed or is it retro active as well?

The plugin only works with blueprints unlocked after the plugin has been installed. I am at some point in the future planning to release an update which will allow you to share all of your current blueprints with another player in the server.