1943 British clasp / Jack knife with black blade?

[Grimebox]

I was given this knife today and thought it looked a bit mucky and rusty, however, after giving it a wipe down with an oily rag it appears that the blade is black. I have not seen a Jack knife with a black blade and wondered whether this is something unusual. It is made by JOSEPH WESTBY SHEFFIELD and dated 1943. The tin opener is silver in colour. It might be that the blade is incredibly tarnished but it is noticeably black when compared to the tin opener and the black colour covers the whole blade (not just the area that sits proud of the handle). The black also doesn't rub off!

[Time Bandit]

I've one or two like that, it's just where the steel has tarnished. They actually look quite cool like that though.

[davejb]

My two are the same its just the way they go after time, i'm not sure if the blade is made from a different steel than the other fitments

[Grimebox]

Cheers for the reply, I thought it would be odd to have a black blade, I guess different qualities of steel tarnish more easily

[Time Bandit]

Steel probably has quite a high carbon content in it as that tends to turn black in time if it's not kept in ideal conditions?