On a MRI abnormal discs appear darker, which is an indication of less hydration than normal disc. The less water contained in the disc the darker the appearance on a MRI. White disc indicates proper hydration or water. A physician should check your L3-L4, L4-L15 and L5-S1.
The dark color doesn't necessarily indicate a herniated or bulged disc. I think SMDeBouse has the best answer, it has to do with the hydration and health of a disc. Dark colored discs indicate a disc with low nutrition and hydration. This is often a case of what is called degenerative disc disease. Discs degenerate with age regardless, but for discs that are wearing out faster than normal it is considered an abnormal condition. This can be caused by many things, including being overweight, an accident, or some kind of genetic influence. In my case it was a combination of an over-pronounced S curve causing increased strain on my lower discs along with an accident where I fell flat on my back from about 5 feet up. In the 8 years since the accident I've had 2 ruptured discs, one of which required surgery. Of the many MRI's I've had, my L4-L5 and L5-S1 discs have always shown up very dark compared to the others, regardless of whether they were herniated.
A degenerated disc is less flexible and resilient. It won't provide as much support, nor will it stand up to as much pressure. It will be more prone to herniation as well. What you need to do is keep your weight down, exercise regularly, eat a healthy diet including lots of vitamin D and consider a reservatrol supplement. Proper motion of the spine will help the discs "breathe" to get new nutrients in and expel the "trash". The healthier you are and the stronger you keep your back the better your discs will be, the less pain you will have, and the less chance you will have of further herniations. I'm currently going through a rehabilitation program with MedEx equipment and I feel better than I have in years. I highly recommend finding somewhere with this type of program.