I don’t know if it’s carbon or something else. I can try whatever so may tip will be appreciated. It’s an old knife of a relative so I just don’t want to throw it away if I can clean it

if it is in fact carbon steel, you'll want to keep it dry and potentially oiled depending on the local humidity, otherwise you'll be constantly battling oxidation.

You could use some 0000 steel wool to get it looking silver-ish again, then washing and drying it soon after each use.

Hey, did you take that from my kitchen drawer? Have a large knife with a similarly discolored blade that was my grandfather's. Baking soda, Bar Keepers, Bon Ami, nothing has worked. Have yet to try steel wool.

Well pls do let me know if you are successful as it was a sentimental value to me as well