why does the negative lead to a solar panel go black,oxidise.?
Pure copper can tarnish to a green coloured copper oxide.

But in the presence of water and contaminants, what happens is more complex, and I've heard it called "black rot" which may be a useful search term.

Black rot seems to be especially associated with salt water environments as discussed in the linked article about wiring on a boat. 

Are you by the coast by any chance? Or in an area where rainfall is subject to contamination by industrial pollution?

In stranded copper wire, damp can penetrate some distance along the wire thanks to capillary action, so you may have to replace a few inches or in extreme cases a considerable length of wire.

The linked thread supports your observation that electrochemistry makes the problem is especially severe on the negative lead of a battery circuit.

This thread on a radio control models group suggests NiCd batteries encourage the same problem, with leaking cells replacing salt water.



Solutions appear to be either using tinned wire to resist the oxidation, or liquid sealants to attempt to exclude the water or prevent capillary wicking into the wire.