Why would it be blue under your eyes?
My sister would get the same thing you are talking about when she was young. It had to do with allergies. It looked like she was drugged or something, she was so blue, and sometimes black. My mom also had the bone cracking feeling along with the blue/blackness under the eyes. This was due to sinus pressure related to allergies and rarely illness. The doctors never were able to give her any help. I hope you are able to get help for your son. Good luck.
I don't known yet which material causes this symptom in my case but I'm doing this blood test and if that won't help, I'll try a diet (with the help of a doctor of course!) to sort out those things that my body badly reacts to. This diet is not much a deal, you simply leave out a few materials/food types from your meals and if your condition change, you know which one it was. You can do it downwards, leaving only a few things first, and if it doesn't work, you leave more and more; or upwards, having a very limited set and adding food one by one. Please be warned that if you do it without the control of a doctor, you might lack some important vitamins and other stuff from your diet and your condition might get worse. So just don't start not eating something without any thinking.