Sounds like a bad picture tube, especially if you've swapped the entire monitor chassis. Even if there was a fault in the wiring going to the game board, you should have been able to get a hazy blue screen by playing with the blue cutoff and drive controls. If you want to be doubly sure, you can swap the pins around in the video input connector - swap blue with red or something, and see if you have any blue then.
Try to find someone local to check your tube with a picture tube tester. You should even be able to bring the monitor down to a TV repair shop and have them test the tube. A picture tube tester usually also has a "rejuvenate" function, to attempt to burn the contaminants off a weak cathode in the tube to restore emissions of a gun. Sometimes it works... sometimes it doesn't.
-Ian