The darkness under the eyes is due to the increased visibility of blood vessels in the very thin skin that surrounds the eyes. It is not a medical problem, and is considered cosmetic although there are some natural factors that can worsen the appearance.
Lack of sleep, weight loss (leading to reduced fat in the sub dermal tissue), fluid retention and normal aging are all involved.
Some things that may help include cutting salt intake down, use a moisturizer with sunblock to reduce impact of age spots and choose an appropriate concealer with the right shade to offset the coloration of the skin. Some families have an increased tendency to this problem and that is not something that you can change.
Some reports say that cosmetic dermatologists may try to treat the underlying vessels with laser therapy if they are prominent however this may have side effects.
do allergies play a part/
Unfortunately the underlying problem is that of anatomy, and this is something that may worsen with time as part of normal aging. Cosmetic agents offer reflective-creams, plumping agents and masking products but cannot remove the fact that there are blood vessels under the skin that become more prominent with time.
Have a look at this site which is a Commercial enterprise and therefore not medically validated: here