A Scanning blind was a device which could overload the sensors of an opponent. It needed to be activated, before it could be used.
Sarah Jane Smith's scanning blind
Sarah Jane Smith wanted to use it against the Dark Horde. She hoped that they would think that she, Rani and Clyde were tougher than Dark Horde. She hoped that they would tell the fleet about it and that they would withdraw. But Sarah Jane forgot her sonic lipstic to activate the scanning blind. That was why she couldn't use it. (TV: Goodbye, Sarah Jane Smith)