It might be an overfill problem with the black. Perhaps the black is only 100% black without the usual 40-50% of the other 3 process colors added in. Or perhaps it is a full 4-color black, but somehow during that fade you mentioned, it turns into just a 1-color black. I know alot of times pre-press artists at print shops will change the black from 1-color to 4-color themselves, but may be careless and not select everything to change, especially if it fades into another color. Hope this helps for future reference.