Well.. this is what I understand about "under the eye darkness."
In reflexology/meridian points. The location under the eyes are the kidneys.
I also have under the eye puffyness. - I am also diabetic.
There most likely is a combination of things which cause that. poor kidneys, poor adrenal glands (endocrine system).
Find a reflexology chart for your feet and other points in the body. Find the ones which are linked to the kidneys and massage them everyday. Drink teas which support the kidneys and adrenals. Also, don't eat sugar, wheat, salt, and milk.
Also use relaxation techniques and get proper sleep. Sleep 10:00, Rise 6:00-7:00.