I think the crash was not visible because it was on the lunar farside, not because it was on the dark side. Remember the moon orbits with the same face toward the earth because it is tidally locked. Both near and far sides experience approx. 28 day night/day cycles. A crash on the farside would not be visible in day or night.

Good for Eugene. The press has gotten in the bad habit about talking about the "dark" side of the moon. They need to put something in their style manuals to the effect that the far side is what we can't see on Earth while the dark side is the side that happens to not be illuminated by the sun at any particular time.

They only said that the mountain was currently dark. Nobody mentioned anything about either the dark side or the far side. They did not say this mountain is permanently dark either. I assume that since they stated that the LRO will be taking pictures of the crash site, it will be sunlit at that time. LRO can't take pictures in the dark. This location could also have just been in the shadow of another mountain at the time of the crash. Things at the poles cast very long shadows.

I would think (I have not calculated the energy release) that the energy released by the impact of two 132kg satellites moving at orbital speeds would create a bright flash that would be detectable by instruments on the earth if the impact sight was on the near-side. The LRO could easily have detected this impact with the LROC (camera)or DLRE (radiometer). Waiting for the orbits of the various spacecraft to sychronize so the LRO could be overhead when the impact happened probably would have exhausted the fuel on the GRAIL spacecraft and they would not have been able to control where the impact occured. That is assuming that the orbital inclination is close enough.