The Short Version
This is a folk myth with no scientific basis. Moonlight is reflected sunlight roughly 400,000 times weaker than direct sunlight — far too feeble to alter steel or drive meaningful oxidation overnight. Peer-reviewed MIT research shows razor blades dull through mechanical microchipping during use, not passive light exposure. No credible scientific study has ever demonstrated that moonlight dulls blades. The only "evidence" cited in support comes from anonymous forum posts proposing physically impossible mechanisms.