While working at CNC engraving, I had at my disposal a small bottle of liquid used to stain brass for creating signs etc - engrave a writing, stain the engraving, then grind/polish the flat topmost layer to remove the colored layer and leave it only in engraved depressions.
The bottle didn't reveal much about composition of the liquid, beyond some warnings about toxicity. It was a brass staining liquid, period.
The coloration it created was dark brown, that seemed black by comparison with the surrounding metal. The reaction took only a few seconds until full pitch of coloration was achieved and the liquid itself was transparent with very mild cyan-blue hue. The coloration was quite persistent; even roughly 10 years afterwards it's still good as new on the pieces I kept and required rather deep brushing to remove from the topmost surfaces.
Can you tell me what substance it was?
(below a sample with dark parts achieved through that liquid)
Edit: I found a very similar liquid on sale, but the page is equally devoid of composition details. The instructions say: [the liquid] is to be used in room temperature. It allows the finish in tones between black and light brown. The time of reaction is almost immediate. The color and speed can be controlled through dilution in water up to 50%.