Why is Live View black with a manual adapted lens, even though the moon photo exposes correctly on a Canon 7D?

If you've got Live View set to "exposure simulation" but the camera is unable to communicate with the lens you'll have issues. That is because "exposure simulation" needs to know 1) at what aperture the lens is currently set (when shooting stills in LV the lens remains wide open until the shutter button is pressed all of the way), 2) at what aperture the lens will be set when the photo is taken. Without any information from the lens the camera can not calculate exposure simulation properly because it doesn't know the difference between the aperture when metering and the aperture when taking the photo.

When shooting the moon it is also possible that you have Live View zoomed in at 5x or 10x magnification on a part of the field of view that does not include the moon. When set to properly expose the moon, it is not likely that any other celestial objects beyond a couple of the very bright planets (Venus and Jupiter) will be visible in LV.

One way to meter and focus with the non-EOS compatible lens would be to switch to video mode for metering. In Live View video mode exposure is based on the amount of light coming through the lens (because with EOS lenses the lens is already stopped down to the "shooting" aperture in LV video mode).

But when shooting the moon there's a much easier way: just use the "lunar 11" rule of thumb. It's similar to the "sunny 16" rule. If your shutter time is the reciprocal of your ISO setting, you should use f/11 to properly expose the illuminated parts of the moon. After all, it is in direct sunlight even though you aren't. Don't forget to add two f-stops to the Av setting on the lens to account for the 2X TC. For an aperture value of f/11 the lens would be set at f/5.6 when using a 2X TC and f/8 if using a 1.4X TC.

For the non-illuminated parts of the moon, please see: How can I capture earthshine?