I put this on the other thread on the same topic - on the Cocker Club website, it says that the "rust" tone to solid blacks is because they have a gold/red recessive gene. It's only black dogs which have recessive genes which are black which end up completely "raven black" - I'd previously assumed that it was the sun too, as grooming does seem to make it better, but turns out it's genetics, although I think the sun will exacerbate it.
Denise