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I believe it's just called Blacken-It but AMMO by Mig also makes something like it...
Uschi also makes a blackening compound (that I have used on brass) They all work as a chemical change to the metal. I found on the brass it turned the brass a dark brownish black and there was a black powdery residue on the surface that needed to be cleaned off.
In the end it worked quite well and the brass for the WWI machine guns looked great! I suspect it will do similar to tracks though I would just paint and weather those.
It seems like I got mine from Sprue Brothers but they don't show it now though there are other Uschi products there.