I have a Nakiri that was very reactive to onions and just about anything else as well. Onions going black and giving off a bad smell was not an option so I just wet an old rag with Rainex( get it at hardware stores in OZ) and let it soak for a few minutes. It took a good bit of the karuchi? finish off and stopped the reaction completely.
The active chemical is Phosphoric acid, it turns the iron into Ferric Phosphate and creates a non reactive surface layer, lasted about 6 months now and no reaction or rusting at all. I do wipe, wash and dry as on all carbon knives though.
Hope it helps.