A great deal of tarnish is sulfur dioxide, that, as several above have indicated, looks a little yellowish before becoming black. By any chance does the water in your area have a high sulfur content? If so, that is the problem. While most of us in the US wouldn't think about sulfur in the water, I remember staying at a state park in PA where you literally smelled like sulfur after taking a shower. Most homes in the area had a sulfur treatment removal hooked up to their water supply.