When I was imprisoned by the Robo mafia, i made my way through the sewer and caught the key from the guard who was chillin on his chair...after that i used it on the box and left the room through the sewer again.
That's where you took the wrong turn. Once you use the key to activate the two green lights go in the room to the left, NOT back into the sewer.
This didnt work for me - the robot shook his head! so I thought I have to go back...i mean where else could i go!?
"the robot shook his head"... at what point, exactly? If you would, please, explain step by step what you are doing to end up in "the dark room" so that the developers can replicate this "bug".
Or, better yet, take a look at
Walkthrough image 6 and compare your steps to how the devs designed the game play to pinpoint where you took the path to "the dark room".