i have not had dark "bags" under my eyes as yet, but i know someone who has had them all his life and they are severe on some days.
they seem to be accentuated by lack of sleep. i think when someone sleeps, their metabolism is reduced to an "idle" state. i also think that during this time of metabolic idleness, that there are few toxins being excreted into the blood stream by metabolism, so the kidneys can work on filtering out the previous day's toxins while not having many new ones do deal with.
why are they "purplish" or "bluish"? it must have something to do with the oxygenation of the blood that courses through the bags. since there is most often no cyanosis anywhere else in the body of people with dark bags under their eyes, i would assume that the respiratory and circulatory system is ok (look at your fingernails. if the skin underneath your fingernails is light pink, then it is an indication of a healthy respiratory and circulatory system. if it bluish then there is a problem).
so why would an otherwise healthy person develop a small patch of unhealthy looking blood vessels?
it may be that there is an area under each eye where arteries become veins through capillaries. since capillaries are so narrow, they would easily become clogged with with impurities that the kidneys can not filter at a pace to match the production of the impurities. maybe most of these artery / vein interchanges are buried deeper in the body and that is why it is most apparent on the face under the eyes.
the clogging would result in blood taking a very long time to transit the capillaries of the "bags" and the blood would become deoxygenated and also the stagnant pressure would result in the "ballooning" of the soft skin surrounding the area.
maybe just always get 8 hours of sleep.
other things to consider are that if most of your family is afflicted, and if they live in the same local environment as you, then there could be a common environmental cause like eg. impurities in your tap water.
do you know if other people in your street or local environment also complain of dark bags under their eyes?
truly i have not many ideas.