It had been full dark for 2 hours, there was no residual lightness to the sky. During a full moon, especially here, the sky is not pitch black at night but blue.
No, it was about a 2 second exposure (actually I think it was 1.6 seconds but I can't remember for sure). A 20 second exposure looks like full daylight except for the stars. This picture is brighter than it actually looked out there but it is still extremely bright at this altitude in a full moon. It is plenty bright enough to hike in those hills, which I often do on full moon nights in the summer. The park is actually pretty crowded with people walking around on a nice full moon night. The thin dry air makes it close to twice as bright as a full moon in much of the east. You can read a book in the moonlight with little trouble, at least I can if I have reading glasses. Of course I need them in the daytime too.