A lot of early computer DVD playing was using hardware accelerated decode, with the resulting images bypassing the framebuffer. That means it won't show up on your screenshots, but it wasn't necessarily detecting a a screenshot and blanking the output (although, once software decoding was feasible, that may also have happened).
I remember this. I also remember the colour of the video window was something like 030303 and if you had that colour on any other window (including your wallpaper) you would see the video there as well.
