Darkening of the Tin plating is a known and expected phenomena. This phenomena reduces the solderability of the Tin with time until there is a need to solder using an RA Flux. Several years after manufacturing the solderability will completely diminish.
In spite of this, the electrical conductivity and the physical properties of the conductor (e.g. Tensile Strength) are not affected. In order to retain good solderability for a longer period it is recommended to store Tin plated Copper conductor wire at a temperature below 30 C , relative humidity of less than 30% and to seal the ends of the wire to prevent moisture ingression during storage.
Below please find some articles on this subject.