What surface treatment can turn nickel black?
The article provided by Karl has answered the question with electroplating method, which is not preferred but this is the only answer I have.


The method is based on MIL-P-18317. The electroplating solution consists of $\ce{NiSO4, Ni(NH4)2(SO4)2, ZnSO4}$ and $\ce{NaSCN}$



The black nickle coating are $\ce{NiS}$ and $\ce{ZnS}$.
$\ce{NiS}$ and $\ce{ZnS}$ are deposited by $\ce{M^2+ + H2S -&gt; MS + 2 H+}$.
$\ce{H2S}$ is generated by $\ce{SCN- + 3H+ + 2e- -&gt; H2S + HCN}$

note: Is reducing thiocyanate a standard method to generate sulfide in electroplating ? I know soluble sulfide salts smells horrible, but it is worth the trouble ? not mentioning the cyanide byproduct, in acidic solution.


Metal $\ce{Ni}$ and $\ce{Zn}$ are also deposited $\ce{M(NH3)2^2+ + 2e- -&gt; M + 2NH3}$.
The role of $\ce{NH4+}$ is to generate $\ce{M(NH3)2^2+}$ prior to metal deposition.