Personally i would use pineapple juice over lemon. It is not as corrosive (acidic) as lemon so it will probably work slower, but there is already quite a bit of metal loss on this blade. Make sure the blade is completely submerged in the juice. How long? As long as it takes.
The blade does appear pretty rusty. You will also need to work the blade over regularly (maybe twice a day) with something like a hard toothbrush to help loosen and remove the rust. Be patient. Could take a week or more.
