another product for darkening steel, cad plated bolts and brass/bronze is Brass Black. works for me.
https://www.amazon.com/Birchwood-Casey-Brass-Finish-3-Ounce/dp/B0000C514O/ref=sr_1_3?keywords=brass+black+metal+finish&qid=1565879751&s=gateway&sr=8-3...