I think that all of the answers point to one common problem but not WD 40. Bluing of steel is a difficult process. The bluing salts get weaker with age, the temperature of the steel has to be correct and the grade of the steel varies from gun to gun or part to part. If any of these factors enters into the process, the bluing is affected and the result is eventually a plum looking color.

Actually, bluing is an incorrect term. The process was originally referred to as browning. Basically it is a controlled rusting process that was applied to firearms going back to the 16 or 1700's. The purpose was to remove the shine of the steel on hunting guns and to retard the normal rusting process. It is the combination of chemicals or salts used that causes the gun color to go from a brown to a dark black. Smith and Wesson changed their "bluing" process and this is why the older guns have that deep dark black blue finish that is not seen in the modern guns.

The plum barrel and cylinder condition is a result of an error in one of the process steps. The steel takes on more of the brown color which makes the blue look plum.

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James Redfield
LM #497