Someone I know is in the packaging business and had the following comments on your blade problem.
Metal detectors cannot see through cans or metaized films. Use them up stream of these operations.
A broken blade point may not show up in product for days, months, or years. Pieces may become buried in conveyors or may adhere to any tacky surface. < 1gram and < 1mm can cause lots of trouble.
Base your plan upon a failure modes effects analysis. Use pareto to prioritize the actions.
Look for ways to minimize the number of blades in the operation. Additionally, use only the most robust blades to minimize failures. Ban cheap disposables especially the notched or beak point blades. Perhaps heavy shears or wheel type blades would be good alternatives to box cutters. Pull tabs, pull strings, perforations, adhesives can all be altered to minimize cutting.
Make blades that could fail or be lost control points. If you know when you used the blade you know when the contamination occured.
Create and deploy procedures for actions cased by lost or broken blades based on
FMEA.