Much has been written on this subject. I have researched this topic until I'm blue in the face.
I've written to various entities for information on the best and safest methods. The Copper Inst.
of America sent me a 50 page document. The Metal Finishing Trade Association sent me a 100 page document.
Even though I explained that I was a hobbyist, these documents were more suited to large manufacturing processes.
I'm not trying to blacken file cabinets or tanks! I've heard of every type of chemical that can be used for this purpose.
I've even read about using Egg Yokes! If the yokes don't work, at least you can have them for lunch.
Some keywords kept leaping out at me, no matter what the source; "Extremely Hazardous",
"Lethal Poison- No Known Antidote", "DANGEROUS, Use with extreme caution", "Can cause Death!".
The "Can cause Death" one really caught my eye. I may be an avid modeler, but there are times where even I draw the line.
For years there has been a product sold at most hobby, gun, and jewelry suppliers know as "Blacken-It" I've tried it, and it's not too bad. But my search,
I believe, is over. The products listed below work quite well on darkening or blackening metal parts.
Of particular interest to ship modelers is the "Brass Black" And you don't have to be a chemist to use them. They can be found at Smallparts.com.
This company is also a great source for other materials. Check out their website and see what I mean. I'm also looking for other sources.
The local suppliers in my area have never heard of them. Not even Model Expo has them listed.