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The black on old copper cable is due to sulphide or oxygen reactions. That is why the majority of cables are tinned plated. Old rubber covered cable was very bad for this, as sulphur was used in the process of making rubber. I have found that the only way to solder these old blackened cables is to scrape the black off, ordinary solder flux does not touch it. Acid based (plumbing) flux might clean it off but will lead all sorts of other problems.
The insulated wires in my very cheap Chinese solar garden lights turn black and are impossible to solder. The rain leakage and humidity caused it. My replacement wires do not turn black because they have some quality.