i donno but i think this is typical...they just paint over the main color unless the bumper comes black from the factory and mask off.
anyway good to hear your ordeal is over.
i would've probably removed the bumper after getting it back from the shop and shot it with 7-8 layers of matte automotive grade clearcoat over the black.
PaintScratch.com - Automotive Touch-Up Paint, Spray Cans, Paint Pens
PaintWorld - Automotive Touch Up Paint and Supplies
also check sherwinwilliams automotive paints.