i used kiwi black shoe polish on my old mk3 golf , i think its like 79p a bottle (asda), comes with a sponge top/applicator, perfect for putting on bumpers , wipes off the paint work with a wet cloth if you smudge it and it lasts!.
the golf is now parked off road (no tax or mot), the red paint has faded pink, but the bumpers are still black!