Rust is ferric oxide (Fe2O3); the black stuff is ferrous oxide (FeO). Ferrous oxide has had some of its lust for oxygen satisfied so it is no longer so obsessed with the sexual urge that it spends every night hanging around bars looking for it.... It retains a coat of oil better than an unblued blade.
Ferrous oxide is not really blue. If the layer is just the right thickness, the wavelength of blue light, the blue component of white light penetrates to the steel below and is reflected back at you. By varying the thickness of the coating you can get other colors too, orange to gold mostly. When it's too thick for any of those optical effects it looks black. Black is safest if it's rust prevention you want.
Parkerizing is a similar process to bluing and makes a more porous surface that absorbs oil better than blued steel, so if you keep parkerized steel oiled it's better rust prevention than bluing -- but if you don't, it isn't.
The same basic process, as you've observed, happens with acid foods and fruit juices. It tends to come out as a beautiful mottled surface that displays different colors when held in a good light and turned this way and that. It has character, much more character than a uniformly black "bluing" job.
Frankly, it's not great rust prevention. You still have to keep it oiled. It's pretty, though.
When the ferrous oxide coating first forms it's fragile and you have to be careful to avoid polishing it off for the first few days until it hardens. Even after it's hardened you can still scratch it or wear it off, but it's easy to renew.
If you'd rather have the looks of plain old boring bright steel (what's wrong with you???) then you'll find it easier to polish off the beautiful patina if you do it very soon after it forms.
If you have better taste than that, you can buy cold bluing solution at any gun shop, or you can try cutting a lemon in half and rubbing the blade with it. If you use the knife in the kitchen, though, especially for cutting acid foods like lemons and tomatoes, it'll happen naturally ... few people bother deliberately patinating kitchen knives; they just let it happen.
With a random process like that sometimes it doesn't come out looking so great ... if your knife is starting to develop some patina and doesn't look great, try a lemon or a tomato on it to accelerate the process and get past that stage. Many people rub a new blade with a lemon right off because it's the early stages of patinization that don't look so great -- rub it with a lemon and you have some control over what's happening and you can spread the juice over the whole blade and avoid the early stage when it looks like a bright blade with one or two stains.
-Cougar Allen :{)
P.S. I had to edit this post because I had confused ferric and ferrous oxide and Walt corrected me -- more details below in this thread.
-Cougar :{)
[This message has been edited by Cougar Allen (edited 20 November 1999).]
