Could black-and-white CRT-based television sets be damaged by holding a magnet to the screen?
Your magnet will magnetize the shadow mask and deflect electrons from their intended path to the colour phosphor and cast a bloom on the image.  



The earth's magnetic field will magnetize the shadow mask as well and if the TV is moved you get the same bloom.  

The design of the CRT is such that there is a degaussing coil around the face of the the CRT which applies an alternating, exponentially diminishing magnetic field to remove the bloom.  

This happens when the TV is turned on after being off for a while.  Your actions with with magnet might have been noticed the first time the TV was turned on, but would have become fainter every day.

There is no shadow mask for a black and white TV so the magnet would have deflected the electrons while it was working, but without any lasting effect.