New knife problem :(

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A very nice patina can be forced on this knife just by doing what you started. Get several more of these onions, cut them one at a time into thin slices. wash, wipe and dry the knife between each onion but don't scrub off the color/patina that is forming. after three or four onions the coloring and rust will stop and the finish will stabilize.

Bill's advice is what I did with one of my knives and it worked.
I found red onions to discolor the first couple times but no more after a patina got going.
I cut a pork roast right out of the oven and got a blue-ish patina all over the blade. After that no more onion staining.

 

[stevenStefano]

I stopped using carbon steel knives because I cut loads of onions in my job. The patina might help but if it's the cladding that is reactive it will be tough

 

[chefjohnboyardee]

My ITK 240 is hoot to use. I've had it for two years. It is sharp, cuts what I need it to and the patina is soft and doesn't discolor food. I've done a lot of experimenting with said knife. It is now thinner, smoothed spine and choil. It will probably be the first wa-handle i attempt. As long as I think someone can handle a carbon knife I will keep recommending it to them.