Bleeding with a skull fracture would usually occur immediately following the trauma, and is normally considered a life threatening event; both would show up on a CT scan.

That said, a concussion will frequently not show up on a CT. As you will read through the posts here, common post concussion symptoms can include headache, dizziness, sensitivity to light and noise, insomnia and tinnitus.

Psychological symptoms can include anxiety and depression, attention and memory deficits.

If your overall cognitive functioning hasn't changed since the head bump I agree with Jo*mar and Mark that your general anxiety and insomnia is the likely cause of the dark circles. Are you seeking help for the anxiety disorder?