Copper cable blackening in transit

I have a repeated problem of copper blackening in Cable that is meant for electrical connection in a Motor. The blackening is due to the oxidation property (when moisture is present in atmosphere).The cable drums when transacted from the supplier to my workplace is wound on a wooden drum and both the ends (openings) of the cable length are sealed with caps.
But can we know whether copper has already started to blacken before it is sent from supplier end though he checks at least 5 mtrs at each end.
Any method to stop copper strands in the cable to blacken?
What precautions are to be taken to stop blackening?
If copper is blackened how to remove it effectively