It was hard to be sure in the extremely unsteady seeing, but as far as I could make out, both spots looked very black and very round. I've never seen a moon stand out that blackly before, especially when standing against a dark band. This was while Ganymede was still off Jupiter's limb, ready to transit. I can barely see Enceladus from there. Last night provided good enough transparency for me to clearly see Enceladus and Mimas as the wind died down at 10 p.m.
Turbulence cuts down on magnitude gain.
This is very interesting. It was certainly not Ganymede's shadow. I've never seen a jovian atomspheric feature anywhere near that dark. But the GRS wasn't visible yet at that point. As Europa continued to move west, away from Jupiter's limb, its shadow and the GRS moved west and Ganymede approached the eastern limb of Jupiter, next to make a transit ingress.