That looks like a lot of discolouration to me. Avery has black tips on her flight feathers, but I attribute that to the butchering of her flight feathers when she was clipped. Otherwise, she's completely green all over with virtually no black. Here's a picture of her back I took a few days ago:
She preens quite a lot, and I actually always have wondered if she over preens. But I find in the feathers that are over preened (in particular her primary flight feathers), they take on bronzing opposed to becoming black. If he is over preening, I highly recommend any "Planet Shredder" toys if you're not toy savvy (like myself). All of the toys I have for Avery are actually shredding toys, and she absolutely adores them. I also have two different foraging toys. Otherwise, I think she'd definitely be an over-preener.
I would also recommend a food processor. I recently bought one to make my chop, and oh my goodness it's so easy! It takes 15 minutes of soaking the veggies/fruit to clean them, and then maybe 30 minutes in total to make a month worth of food that is absolutely jam-packed with veggies and fruit. I actually made a fruit chop and a veggie chop, and the birds go crazy over them. I found Avery actually ate less when I hand chopped my chop, and while I do still give her stuff I hand chop, I find she ingests way more good stuff with the processor.
Perhaps you could make a fruit based mash with lots of yummy berries and then add in some veggies high in vitamin A (carrots, sweet potato, sweet red peppers, kale) with some fruits that are also high in vitamin A (canteloupe, mango, apricots). It would probably help with feather colour and help get rid of those black tips by his next moult or two.
