Just answered my own question. Set the refractive index to 100 and the dark spots go away. These are PE TA renders. Just to prove the point, here’s a render with no dark spots and no refraction in the glass. It’s just a WIP not a contest entry (not that it matters, just trying to make Pam’s job easier)
I’m guessing your ‘dark spots’ problem is down to the render settings. If you re-apply the refraction to the glass and then in your render settings increase the Maximum Ray Depth to 8 and maybe turn Total Internal Reflection on and set it to 2 or 3, the dark spots will go away and you can have the glass refracting in a more natural way.
Hope this helps.