I have several firearms that are electroless nickeled with the high-phosphorous (more corrosion resistant) version. So applying any electroless nickel finish that is itself charcoal gray or black cannot occur.

Does Duracote® or other similar bake-on dark finish adhere to electroless nickel?

If this family of bake-on dark finishes cannot adhere to electroless nickel, what relatively durable dark finish does?

I think there are black nickel finishes out there, but yes, you can paint over electroless nickel; sand blast it first, which you have to do anyway. Electroless nickel is not super hard so it will be etched by the sandblasting.