couple bits of questionable info here.
Quals for this post. I'm a textile engineer, with backgrounds producing ballistic nylon, as well as making tennis ball felts *which is a dyed wool and nylon blend*
The bladder is made of ballistic nylon. Nylon is meant to be used as a synthetic version of wool and was specifically designed to use the same dye stocks as wool so blends can be made without having to go through two dye baths. Wet finishing is horrifically expensive, so it's very important that they dye similarly.
Black wool dye, with all steps followed should get the nylon back to jet black.
Now, reasons nylon fades. Nylon, like the rest of the Amide family *Nomex, Kevlar, etc* are all pansies. They are very strong, but they don't like any chemicals, and they don't like sun. Breakdown occurs fairly rapidly in warm chlorinated environments, and pretty quick in the sun. Some wings have been made with PET for their resistance to chlorine for pool use, but sacrifice abrasion resistance.
As the fibers start to break down, they let out their dye which means their strength and abrasion resistance are fading as well. For the Halcyon outer covers, the only thing you risk damaging is the zipper, but it shouldn't have an issue in the dye bath.
Me personally? faded gear just means it's been well used and if you're going to use it, it isn't going to stay looking pretty for very long no matter how well you try to take care of it
