Onions didn’t work fast enough in putting a patina on that $11 Mora Classic and I was thinking why didn’t the patina I expected develop? ... warmth and humidity.  I was on a South Pacific island when this technique worked so well.  So I warmed the blade over my kettle while boiling water for coffee.  The blade became warm and had warm water on it.  Then I wrapped it in a paper towel soaked in a little apple cider vinegar, set it aside and drank my coffee.  When I saw it again the paper towel had a reddish discoloration which was concerning, but it was just the paper.  
The steel had the gray patina I was hoping for, so I washed it, dried it and then treated it with   
Camellia Seed Oil, which is also referred to as 
Camellia oleifera and 
Tea Seed Oil (
not Tea Tree Oil).  From what I’ve read, it’s food grade so should be suitable for a carbon steel knife used in preparing food and is also used on woodworking tools in Japan.    FWIW, $.02