I tried the Birchwood-Casey Copper/Brass Blackener on one that I recently refinished. It came out so-so and it's a real B-I-T-C-H to work with.
The bottle states to apply, let sit for 1 minute, then wash off. I tried those directions and it barely made any difference. So, I tried leaving sit for longer. While it darkened it a little more, it still wasn't close to the original finish.
It took 10 separate 'coats' to get it remotely close enough to call it a day.
After each coat, I very lightly hit it with 0000 steel wool, which made the finish look a little better, but still needed a deeper black.
There may be other products that do the same, but the Birchwood that I used was not all the great.
I did consider using the Duracoat product as it seems that is pretty good, but the cost of over $80 per spray can was a turn off. Plus it requires other products for prep.
Looking back, it might have been a better option?
Mike