It's to do with genetics. This is a quote from the website of The Cocker Spaniel Club -
"Genetically there are several different variations of "black" as we see it in a dog. In reality there are blacks that are genetically pure for this colour and as the most dominant colour, they will only ever produce black coloured offspring, although these offspring may be blacks carrying a different genetic mix. The genetically pure blacks tend to have raven black hair. Blacks carrying liver, red in either variety of pigment, golden and of these, those which may or may not carry the gene for tan mask and trim, may appear to be black but may sometimes show a touch of "rust" in their coats."
I think the rustiness will be more noticeable in a coat that isn't groomed frequently and contains dead hair, or maybe in a dog/bitch that has been neutered as that often changes the coat a lot.