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I'm ASSUMING your opie is a carbon steel blade...
Do it naturally, i.e. food prep. I patina'd my opie with vinegar, only takes minutes of exposure, and it did give a deep dark grey patina. I thought it kinda ugly though.
I have a recently acquired knife with 51-200 steel, and have used it to cut meat, veggies and fruit of all kinds. I used it normally, and cleaned it fairly promptly with mild soap and water. It has developed a most kick-a$$ multi-colored patina, lots of purples, reds, oranges, blues. Looks WAY better than a forced patina, i.e. sticking into a potato or apple or soaking in warm vinegar etc.
I used some mango salso we had, made from our mango tree, and the mango didnt seem to do a whole lot, but something in the salsa turned black and stained the metal in a black blotchy outline where it was. I think it was the onions... It looks cool though.Mango is the quickest. It's like instant patina.