Rust is steel’s number one enemy, and if a blade is put a way wet or neglected for too long, rust can find its way in. If the rust is minor–just a haze, or a few tiny specs–make a thick paste of water and baking soda to use as an abrasive. Carefully scrub the affected area to remove the rust. (NOTE: if your blade is damascus steel, this process may also remove some of the pattern’s contrast. While the pattern itself is intrinsic, the contrast is caused by a thin black iron oxide layer on the surface of the steel. Rust is red iron oxide, and I do not know of any way to remove one without the other.)
If your blade is more severely rusted, please contact me right away and arrange to ship your knife back to the studio for more thorough restoration. Depending on the extent of the corrosion, there may be a charge for this service as it will likely involve completely refinishing the steel.