Question about a seller's claim of a 'factory blued vintage' Sharpfinger

Bluing, as used on firearms, is "basically" chemically induced rusting. The salts used in the bath start the rusting process, which is then halted. This gives the metal a thin layer of protection because it's much harder for the natural process to start again. If the salts are not entirely removed on taking the metal out of the bath the metal will continue to rust. This is usually done by boiling and then coating the parts in oil which removes the salts and then denies them access to oxygen and halts their ability to change the metal composition. This is why certain metals don't "take" bluing as well as others. Metals with high Nickle content (for example) turn purple.

"Cold" bluing uses a slightly different set of chemicals, but does the same sort of thing. It's not as good, and actually works best when applied to metals that are warm (not so hot you can't touch them). You can tell the difference in cold blued parts, if it was recently done, just by the smell.

The older method of bluing is to actually rust the metal in a humid chamber and to card off the rust as it develops. Often called rust bluing, this is very time consuming, and it's what was used for centuries before the salt baths became commercially viable. This is what gives guns from before WWII the deep blue color and it it still done for very expensive custom jobs.

Neither of these will totally stop rust. Humidity and chemicals in the environment can cause the rusting to continue.

Coatings, such as parkerizing, are basically designed to prevent the bare metal from coming in contact with oxygen and starting to rust.

Steels can also be blued by changes in heat. Anyone who's ever heated a piece of metal has seen this. The color varies based on the very specific temperature. This is commonly used for small parts (Colt screws on pre-war handguns were had heat blued.)

If properly done a blued knife is no less usable than any other. The patina so beloved by some is nothing more than the same rusting taking place with the chemicals the steel has been exposed to. Chemical bluing is toxic, but if done properly the chemicals are removed from contact with the steel once the bluing has reached a certain point. Once they are gone the metal is no more toxic than the environment it is in. There are blue steel knives produced commercially in Germany and Japan specifically for kitchen use.

This is pretty basic, and unless you're going to start bluing something probably way more info than needed. But if you're worried about using a blade with food, boil it, and then coat it with oil (WD-40 works very well, as it's a water displacer), let it sit for 24 hours, remove the oil (hot water usually works well), and then "slice away".

Back to the OP, a used Sharpfinger can be had for $5. If you want a blue one make your own.
 
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