Handmade Knife

I have a knife with black…smudge?

it’s not rust from what I know because it’s not rough its smooth but its all over the blade and its all dingy what is it? and is there a way to remove it and make the blade shiny again.
ps. I don’t know what kind of steel it is, I didn’t buy it it was handmade for my uncle who gave it to me

If it was handmade and it adds character, leave it

Hand made knife making project in 3 hours