There are a couple of versions of this story. I first heard it when I attended RCA Broadcast technical training in the 1970's. Here's the basic story: In the 1950's when color television was being developed by RCA, the engineers would often use a bowl of fruit as a test subject for the cameras. One day one of the engineers painted the banana blue. Another engineer worked feverishly to try to make it look yellow. He finally succeeded, but the apples and oranges were all wrong.
Prior to going into broadcast engineering I worked with the engineer who was on duty the night the "blue bananas" prank was pulled.
It was to be the first color transmission ever and the studio engineering staff was working to make sure that it went perfectly. At that time the cameras were huge and they started there to set then up. They used a still-life bowl of fruit as the subject. After completing the camera setup, they moved to the control room the setup the camera control units (CCU's). After setting the phase of the I & Q signals - the quadrature phasing which adjusts the hue - they put the picture on a monitor. The apples were red, the grapes were green, the oranges were orange and the bananas were blue.
Now in a panic with only a couple of hours before air time, they readjusted the CCU's from +/- 1 degree to +/- nothing. The damn bananas were still blue. Frantically they went into the studios to recheck the cameras. That's when they found that some one had painted the bananas blue.