I have had a dark circle under my right eye since I was 17. (I am 30 now)
I have been looking for a fix for quite some time. Spoke to a MB councellor recently and he indicated it to be a kidney thing. I also have other symptoms like back ache in the kidney region, anxiety etc and all point to the kidney region.

Also, there's information about dark circles/eye bags in the Cancer prevention
Diet by Michio Kushi (In the kidney cancer section). Another interesting book that pointed me in this direction was the book 'Your face never lies' (its about facial/body reading for diagnosis)

I have been on the MB diet for only a month so far.. The dark circle hasnt disappeared yet.. just keeping my fingers crossed