When I first started getting into Japanese kitchen knives I was very interested in carbon knives and how they did or didn't react to specific foods. I didn't want to taste steel on fresh fruit, know what I mean? Yuck!
Over time it's become clear to me that carbon knives (once "seasoned") are very safe to use on anything that's going to be cooked (like onions for example) but still may not be the best choice for fresh fruit where I would select a knife with at least stainless steel cladding over a carbon core if not full stainless. Carbon knives appear best used for proteins, both raw and cooked, as they seem to impart no flavor to the food. Anyone notice what steels are predominatly used for sushi knives? Yup - carbon!
So even though we know that we can use them for some tasks safely there's still an issue of maintenance that's going to be greater than that of their stainless cousins. Use and care of a carbon knife is somewhat like that of using and caring for a cast iron pan - they need some TLC in the way of seasoning to keep them in top form and to keep the red stuff (aka- rust) at bay.
Camellia Oil for this simply because it's made for the task and also washes off easily. Once the surface of the knife begins to oxidize across it's entire length (that is forms a patina) then the need for oiling is greatly reduced. I will use the oil until the knife crosses over from brown patina to blue, purple, black(ish) as this is often an indication of a nice strong seasoning layer having been formed. I would strongly advise, however, that if you are living in an extremely humid environment (such as Hawaii) that you always coat your carbon knives with oil before storing away.
Remember,....patina = good....rust = bad!
There's a fine line between the two and you're the one who controls it.