Remember, the actual "dark side" of the Moon is not the far side, facing away from us that we can't see, but rather it's the side that faces away from the Sun that is illuminating it.
How dark is the side of the Moon facing away from the Sun? ( I don't mean the far side)
1$\begingroup$ In what units are you expecting your answer? Do you mean how dark is it if you are there or how dark is it if you were to view it from some distance away? Or do you mean how dark is the dark side as viewed from Earth? $\endgroup$– ProfRobNov 9, 2022 at 9:57
It depends on whether the part facing away from the sun is facing towards the Earth, or not.
If you are on the near-side, and the Earth is above the horizon, there is quite a lot of light from the Earth. The Earth is big and reflects much more sunlight than the moon does.
But if you are in the lunar night and you are on the far side of the moon, the only light comes from stars, and it is very dark, too dark to see anything.
1$\begingroup$ Not only is the Earth much bigger than is the Moon, it has a much higher albedo than does the Moon. On the near side of the Moon during a new Moon (as seen from the Earth) it will easily be bright enough to see lots of stuff because a nearly new Moon (as seen from the Earth) means a nearly full Earth (as seen from the Moon). $\endgroup$ Nov 9, 2022 at 7:24
$\begingroup$ @James K. You say that someone can't see anything by starlight. Possibly you have poor night vision and think that other people can't see anything by starlight. I think that some people would be able to see where they are walking by starlight after their eyes adapted to the dark. $\endgroup$ Nov 9, 2022 at 18:32
$\begingroup$ Possibly. When I've been in places on Earth lit only by starlight I've not been able to see anything, except stars. $\endgroup$– James KNov 9, 2022 at 20:43
$\begingroup$ According to the current Wikipedia entry for "earthshine," earthlight has a calculated maximum apparent magnitude of −17.7 as viewed from the Moon. Earthshine has a calculated maximum apparent magnitude of −3.69 as viewed from Earth. The entry for "planetshine" currently provides a generic diagram: en.wikipedia.org/wiki/Planetshine#/media/… $\endgroup$– WPWPWPNov 20, 2022 at 2:05