How often does she preen those feathers? Over preened feathers sometimes turn black or grey over time because they become stripped. Otherwise it could be dietary, from perhaps a lack of calcium or protein. The best thing to do would be to see an avian vet and to have a blood panel done to see where her mineral and vitamin levels are. I'm wondering if perhaps the Mazuri pellets would be better switched out for a better quality pellet, like Harrison's, TOPs, Golden'obles, or Roudy.
Over what period of time did the feathers change color? Does she ever get any natural sunlight?
If you google B&G macaw flight feathers (under images) you'll see what they're 'supposed' to look like. I'd suggest you open up Demi's wings and compare, so you can see if there is a huge difference.