Thanks to all who have replied. I suppose my last question is, does black spotting like this eventually turn into red rust if not removed? Or is red rust something altogether different? If it's different, and the black spotting you see in the pic isn't harmful, I'll probably just leave it. Thanks.
It's not easy to tell from your photo but it doesn't look serious to me. The grind marks appear to be much deeper than the oxidation and it looks almost as if the oxidation has taken place along the grinding marks -- which is more or less what I'd expect but I don't want to see what I expect if it's not really there.
For rust to form you need iron, oxygen and water. If you keep the knife in a dry environment (I think the target humidity figure is less than 38%RH), and especially if you oil it (which keeps water away from the steel), you shouldn't see any further oxidation at all.
Any contaminant or microscopic discontinuity in the surface will help corrosion to start and will accelerate it. The black spots are no exception. I've seen welds that I've made in mild steel stay as free from corrosion as if it were stainless for months, even in a fairly damp atmosphere, just because the weld happened to be very smooth and clean after the slag was chipped off. But if you just touch the surface with something you scratch and/or contaminate it, and then the rust starts and proceeds very quickly. Whether you patinate a surface or even paint it, you're hoping to make a perfectly smooth unblemished surface coating. Of course that's impossible, but as has been suggested you can have, er, a stab at it with items from the supermarket...
Shaggystu made a good point about sharpening a blade which has no secondary bevel. I'd add that the edge of the blade is one of its biggest discontinuities, therefore it's one of the places most at risk of corrosion. So that's a part that I like to take special care of. In your photo (again, I think) you can see oxidation on the edge, it will probably disappear if you give it a light stropping so you'll get a rough idea of how deep it is and how much work will be needed to get rid of it all. Not much I guess. Have you sharpened it a little yourself already?
As others haven't quite said, the best thing to do with it is to keep getting it out of its sheath and using it. You can tell people it's called 'routine maintenance' or something like that.
