I had read once that dark circles are mainly tied to kidney health. I do know that when I started doing things for my kidneys, I experienced lightening in the darkness (like a racoon).
I now believe that it is indeed tied to dehydration, and that should be something you direct your energies towards. Such as using a good mineral salt with distilled water. Eating as much Watermelon as you can get your hands on, or perhaps finding and using Watermelon seed tea. Also parsley is good for kidney health, and you can also steep that into a tea.
I know that allergies can also play a part in dark circles. As far as I can tell, OP is great for allergies.