As promised, here are some links.
https://www.uschivdr.com/products-in-detail/friul-burnishing/
https://www.amazon.com/Novacan-Black-Patina-for-Solder/dp/B003WH3SIE
https://www.megahobby.com/products/metal-burnishing-fluid-100ml-bottle-ak-interactive.html
https://freetimehobbies.com/ammo-by-mig-jimenez-metallic-tracks-burnishing-fluid/
Like I said before, I wouldn't waste my money on Blacken It unless you're working with brass, even then, the results are marginal.
Several articles have been posted on the forum about how to use these products, use caution and ventilation, not to mention safety glasses and gloves (yes, even if it says you don't need to).
Guess I should also mention there are many more places to buy these products, these were the first to come up when I Google searched for them.
:drinks