How did old television screens with a light grey phosphor create the darker contrast parts of the display?
Forget old fashioned TVs, have you ever watched a PowerPoint in a normally lit conference room? The projector screen is white until you turn on the projector, at which point it seems to become dark grey.
This is because your eyes are sensitive to relative light levels and not absolute. Black doesn't require no light, it's just less light. You can can make a white object dark just by putting something brighter next to it. Projectors, LCDs and CRTs of all kinds exploit this effect.
It's basically an optical illusion. I never ever remember anything but a very dark screen when I was a kid waiting for "the box" to warm up and then my mum shouting that the TV needed some money in the slot to make it work (damn those rental TVs in the 1960s).
Consider this picture (taken from this wiki site): -
It looks like the "A" square is darker than the "B" square doesn't it?
But if I use paint.exe to sample A's colour and draw a line from A to B we see this: -
You don't have to believe me. Just copy the top image and try it yourself.
Your brain can play some amazing tricks in trying to make sense of the optical info it receives.
It was not watched in as bright ambient light as a good photo of the tv receiver needs when the camera was supported manually. Or the photo was taken with long exposure time having the camera on a tripod. The old CRT television was placed to as dark corner as possible for good contrast. I have had a CRT B&W television and it was useless if the daylight in the room wasn't dimmed by drawing the curtains in front of the window.
Other aspects: The advertisement photos from 1970's can be heavily retouched for the appearance that was then thought to be the most attractive. They can even have "for ads only" dummy CRT or an inserted mask for easy good looking no-reflection photographing. I guess too dark screen could be seen as "it will stay dark".
I have just now one CRT from 1980's in front of me. I brought it out from the storage only to see it due this case. It doesn't look grey, it's a little green. It's B&W phosphor glows white, not green and it looks substantially darker than CRTs in questioner's ads.