[Zenka]

Isn't it the dark side of the moon, since spacecraft passing behind the moon go 'dark'; out of radio contact since the moon blocks radio signals?

On the other hand: it's the side of the moon that never receives earthshine. So it's dark in that sense.

[sdenton4]

Which is a great reason to get a dumb little cubesat comms relay into lunar orbit...

[Beltiras]

I hadn't thought about that sense of the phrase. That makes total sense.