Cold blues contain an acid that can actually promote rust.
Also, cold blues tend to leave a discolored blotch in the original blue around the area where it's applied. This blotch is permanent and can make it look worse.
Cold blues are not durable and can be wiped off with your finger.
Cold blues also tend to turn brown.
The Express finish is a bead blasted blue finish.
If you use cold blue use an artist's brush to apply it to just the actual spot and try not to allow it outside of that to prevent the blotching.
Rinse VERY well with hot water and apply a good lube. Watch it closely to insure it doesn't start rusting.