That's a very nice sgian dubh. I'd almost think it's civilian rather than military in origin (but I could be wrong).
I'll let someone else speak to rather the blade is original or not, but I will join the chorus & say do not replace it! I'd even be hesitant about cleaning/buffing it too much.
I had a friend who once found some antique (& very tarnished) silver spoons in an abandoned house. He took them home, decided to spiff them up, thinking that shiny they'd be worth more only to later find out that by cleaning them he reduced their value to but a wee fraction of what they would've been tarnished