How do make the surface of non-standard screws turn black？

【High Quality Stainless】These Flat Head Screws are made of 18-8 (304) Stainless Steel and have Black Oxide Coating which is one of the highest forms

Bolt Dropper 6 X 1'' Black Oxide Coated Stainless Flat Head Phillips Wood Screw, (25 pc), 18-8 (304) Stainless Steel Screws Corrosion Resistant Flat

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