What Causes Darkening Of Two Lumbar Vertebrae?

Hi, I recently had an MRI scan for lower back pain, and the results show two of the lumbar vertebrae as a dark black compared to the other white and bright ones. my consultant tried to put it down to bad posture or maybe bad genes. surely there is a more solid explanation as to what this is or how it occurred. I'm only 25 and am otherwise fit and healthy. thank you