How to turn 18K gold to a glossy black colour?

Dear Experts,
I am a jeweller and have been asked to make a cuff style bangle in 18K gold. The bangle is a simple smooth cuff shape 11 cm long (along arm) and with a glossy black finish around a motif. As it is a bangle it may be sprung open slightly to fit on the wrist, and it will need to withstand knocking that is inevitable with cuffs. Can you please recommend how I can create a fit for purpose glossy black surface on my 18K cuff.
Thanks for any advice you can give.
Andrew,
Penang, Malaysia

1.
Oil it with linseed oil(very,very slightly),then heat it to 200- 400 °C (slowly).Repeat if needs!

2.
copper or silver plate it and then colour it with liver of sulfur ⇦ on eBay or Amazon [affil link] solution

3.
plate it with black nickel or black chromium or black palladium

Hope it helps and good luck!