Oh, so that's what patina means...! I'm a big Opinel fan myself, mostly for cutting food, and I've noticed this protective dark layer it would develop after cutting limes. Well, that's exactly what I would do before putting it away. Of course, I'd have to repeat it each time after sharpening it, which is quite often, as the carbon steel tends to lose its edge rather quickly. But I still prefer it to the stainless steel, because you can really get it sharper than any other knife. I'll try the hot vinegar method though. The patina in your photo looks really good.