I bought this George Forman Grill with detachable trays that are supposed to be dishwasher safe. I noticed that after the dishwasher there's this black soot on them. What causes this? Is there anyway of preventing it? The drip tray that came with the grill has written on it "wash in top rack only", should the same be done with actual grills? What difference does it make if something is washed in the top or bottom rack?