A: Dark circles under the eyes could be related to a lot of things. Most commonly they could be related to sleep deprivation. They could be related to adrenal issues. They could be related to allergies. Mold can cause dark circles under the eyes if you've been exposed to that, as well as iron deficiency anemia, and vitamin K deficiency.
You just asked the question about dry eyes and dry mouth, and I suggested doing some omega 3s. I'm not sure this is relevant for you, but in the big picture of people with fat malabsorption issues, having dry skin, dry eyes, dry mouth, as well as dark circles under the eyes could be due to fat malabsorption.
Essentially, this is manifested by the fact that you're not absorbing fats correctly. Therefore, you're going to be deficient in the fat soluble vitamins, such as A, D, E, and K, which specifically goes with easy bruising and with dark circles under the eyes. There are some things you can definitely do for that; you can take some digestive enzymes to support yourself. Pancreatic enzymes can be helpful, and liver and gallbladder support can be helpful. On my website, I also have a whole article about fat malabsorption and some of its consequences.