Filler knife turning black

[Raj]

Hello,

I recently bought a Harris Filling Knife but after I washed it in warm water, some brown spots formed on the tip of the blade. I thought I could leave it in white vinegar over night to remove it but now a black layer has formed over a quarter of the blade and now I am not sure what to do to get it back to it normal!

Can someone help!

Thanks,

Raj