What makes this article super easy to write, is that a rust eraser works exactly like a real eraser. Be sure to stroke back to front when you're working near the blade. If you don't, the blade will shave the eraser.
I haven't really seen these around, but I got mine at the House of Knives. Perhaps other knife stores carry them.
A note on rust - rust isn't just something that grows on the surface of the metal. It actually eats the metal, digging slowly as the top layers come off. If you see rust, you'd best clean it as soon as you can. Rust is usually caused by water, but sometimes moisture in the air (humidity) can also cause it. If salt is added to the mix, reactions speed up.
Since it's an eraser, just rub it over the surface of the metal. Wipe / blow away the shavings that come off and keep working until you get rid of all the rust.
Just look at that photo! I've cleaned half the blade and not just the rust is removed but the metal is also cleaned. Incredible!
As a safety precaution, place some electrical tape over the blade's edge when you're cleaning the other end. (Since you'll have to hold the sharp end)
If you choose to do this, you're probably best to clean the back first, then the front. Then you can remove any stickyness left behind by the tape.