Better tip, warm up vinegar to almost a simmer, pour into a glass and set your knife in there point down, you'll get a patina in a fraction of the time, and a harder set one, since the heat causes the steel to expand slightly, letting the patina set in deeper. Plus, you can control the patina just by watching it.

Should say, I did a couple Opinel carbon knives this way, they turned really black in about two mins. Most I do, such as O-1 steel knives I make, takes 10 mins to get a dull gray colour.

Experiment, and have fun!