Black coating for nickel

We currently plate nickel on the back side of a single sided chip carrier (it is non-functional other than withstanding heat and atmospheric tarnishing). We now have a customer who requires that the 'back side' be black. What options are there for putting a black coating on nickel or base copper (1% Fe + Si). The front side is essentially a build-up PCB, and so copper oxide is not acceptable (too corrosive to the dielectrics and circuitry on the front side). We need a black coating which is 'kind' to epoxy/acrylic, with withstand heat and humidity (e.g. 168 hours at 2 bar pressure cooker test), and will not scratch too easily.

Thank you in advance for the help.

Black Nickel (Metal Finishing Magazine, November, 1943) over 3 tenths of nickel or cadmium over your copper base may work.

I know that the following works on electroless nickel. It may be too aggressive on electrolytic nickel so modification and caution are advised. 140 F 35% nitric Preload with 6,000-10,000 ppm nickel Immerse just to black. Will remove some nickel.