It's not beyond recovery.
Assuming the blade is pitted (looks like it is), the pitting could be sanded out, or mostly so, with a block with wet/dry sandpaper wrapped around it, and the blade supported against a firm backing. OR, just lay 1/3 or 1/2 a sheet of wet/dry over a hard, flat surface (glass, stone, hardwood) and move the blade against that. I used a similar method to thin & polish blades on two of my Opinels. The 'carbone' blades will sand easily, and some 220-400 grit wet/dry paper should work pretty fast. Might start with the 400; if that turns out to be a little slow, move back to 320/220 and try again. You don't necessarily need to remove all the pitting, if you don't want to. Take the wet/dry as far as you like in grit; anything up to about ~600-800 will leave a finer satin finish, and 1000+ will start to bring up a mirror shine.
If the pitting doesn't bother you that much, you could just scrub the blade with WD-40 or mineral oil, and steel wool or a wire brush, to get rid of any red rust. The dark grey/black patina won't harm the steel at all, so you needn't worry about that. After scrubbing the red rust away, clean the blade and use some metal polish like Flitz/Simichrome to shine it up a bit.
Might wait a bit for more suggestions from others as well. Lots of Opinel fans here, and you should get some good feedback.
David