Black nickel is various black substances containing nickel:
Nickel_electroplating#Black_nickel, a dark coating applied by plating; Electroless nickel-phosphorus plating with a black surface finish (black electroless nickel) Nickel oxide hydroxide, NiO(OH), a black solid used in various battery chemistries; Nickel(III) oxide, Ni2O3, also known as black nickel oxide, an unstable and rare compound.
See also
Bluing (steel), a blue-black conversion coating on iron Black chrome, a durable chromium plating sometimes called "black nickel" that resembles black nickel, but is much harder Phosphate conversion coating, a matte grey/black conversion finish