An E.F.S Blocker is an advanced piece of rare technology.
The E.F.S Blocker was developed during the war as a way to bypass the E.F.S spell. The device renders the user invisible to a PipBuck or other device with a built-in E.F.S spell, allowing them to sneak up on E.F.S users. The device does not render it's user invisible or silent though, meaning they can still be seen and heard.
Barb's shades were given one of these so they could spy on Protégé and look through his files for information. Protégé's eyepiece has a built-in E.F.S hence the need for the device. The E.F.S Blocker was taken by Murky Number Seven who later used it to sneak into the Funbarn and rescue Protégé.