[QUOTE=Surly] The black finish is not as durable, typically. As a result, the black will wear and show brassing more rapidly than a silver finished camera. QUOTE]
To add to the above comment, a contact of mine who was in the photographic retail trade once told me how he'd sold a black bodied 35 SLR to a customer. A few days later, the customer re-appeared to buy a case. He had the SLR with him, but when he produced it, it was showing brassing around all the usual wear points to the degree one would expect to see after years of use. On questioning, the customer admitted that he'd whiled away an evening with a bit of fine abrasive paper and some Brasso trying to give the new camera the kind of worn look that it might have gained with heavy use in the hands of a pro.
On a different note, I had reason to take apart my black Pentax ME some years ago. I'd always been under the impression that where the black coating had worn off, I was seeing brass underneath, but on taking the top off it was immediately apparent that there wasn't a bit of metal in sight. The whole top was plastic, carefully coloured to look like brass when its coating wore off on the corners.