You obviously have a carbon steel(non-stainless) blade for that to happen. The acetic acid in the fruit has essentially etched(and oxidized) your blade. How deeply it did that depends on the steel. It Can be re-polished(I would charge you since it is non-stainlesss), but the fact of the matter is, it will eventually discolor if used a lot anyway. Personally, I like my carbon steel blades better as they "age" and pick up that grey/black/blue "patina". Trying to keep it bright and shiney forever is going to be a frustrating(read probably impossible) job. Its a bit late to try one of the various hi tech coatings out there( I think) that might prevent discoloration, but then some of them are uglier than what you have now. I've been using a knife that I made from A-2 high carbon(non-stainless) and that I cold blued. That looks good temporarily, but with hard use, will wear off rather quickly. At that point the blades is free to pick its own color scheme......long as it has no rust, I'm happy with it.
P.S. Polishing it yourself would involve using emory paper in grits from around 320 to around 1500 to 2000. I've no idea what level of polish was on the blade to begin with. Automotive body shops carry the finer grits, some walmarts have them in their automotive sections, All else fails, yo can get them from Knife Supply Catalogs. The 320 is about the finest grit you can use that will actually remove some metal(in order to get the etched portion off). All the other grits simply polish. It's a matter of rubbing the blade with strips of emory, backed by some hard material, until the marks from the previous grit are gone. This can take time and patience to do. Each strip should be at least as wide as your blade so the polishing is done over the whole surface simultaneously. I hand polish all my blades to a mirror finish, and my grit sequence usually runs like: 320/400/600/800/1000/1200/1500 and maybe 2000. As you proceed with the finer grits, you will notice that everytime you touch the blade, or stop the paper on the blade, it leave a slight "smudge" mark. I take those off with a buffer and green, then white, then pink buffing compounds, but it can be done by hand by doing the final paper strokes lightly and full length of the blade, starting at the rear of the blade and stroking forward. Also helps to use tiny amounts of some oil on the blade when polishing.....I use plain old olive oil, but any very fine grade of cutting oil also works. After you do all that, you may slow down further discoloration by coating the blade with a good hard wax like Johnsons automotive........But eventually you will get discoloration.
