I specialize in making hand hammered serving dishes and I get my patinas using successive coats of vegetable oil to create a food safe / moisture resistant finish.
The secret here is to paint the piece with oil in thin coats - not dip it. This is especially important with thin sheet stock as immersing it cools the metal too quickly for the oil to carbonize and it leaves it with a translucent soft coating.
Heat your piece in the forge to between 500 - 800 degrees - Use a natural fibre cloth (don't use polyester blends) dipped lightly in oil.
Bring your piece out of the forge and begin painting it on the surface (it will smoke big time - make sure you have lots o' ventilation and it'll flash at the higher temps so be careful!) but continue to paint it on in light coats until the metal starts to cool and accept it - the cloth will carbonize and add to the black color of the patina. Continue this until you get the result you're looking for - If need be, reinsert the piece in the forge until the existing oil starts to smoke - remove it and continue.
This technique works equally well with any oil. For my purposes I hand sand and buff the piece to get the hammered texture to show back (see attached pics) I then wash it down with rubbing alcohol to remove any residue. If it's going to be a food contact piece, I rub on a thin coat of vegetable oil - If not, I use a satin coat spray lacquer. No worries about the veggie or olive oil going rancid. The oil is thoroughly cooked (like cast iron cookware) and even a light coat of fresh oil is ok because unlike wood, the metal isn't absorbing a deep layer of oil that lies there and rots over time.
Soaking in Muriatic acid to remove scale and etch the surface really helps for creating a consistent patina.
Because I don't paint or powder coat my work, I always tell my clients that my pieces are moisture resistant - not water proof. It will rust if left to the elements - I instruct them how to clean up light rust with triple-aught steel wool that's been dipped in vegetable oil.
It's been a great technique for me - I've been using it for years and never had a return or complaint due to the finish on a piece failing.
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