If the black spots are greasy and require scrubbing to remove then the culprit is probably the thick grease liner inside the door. Basically it is a 1/4 thick piece of laminated GREASE. It is glued on the inside of the door. As the dishwasher gets used, the heat of the dishwasher cause the sheet of grease to slide down the inside of the door and eventually slides into the dishwasher. As you open and close the door piece of the the grease sheet break off and dissolve into little spots every time you run a load. Remove the inside cover of the door and cut (with a box cutter) the sheet off so that it is not being broken by the motion of the door. Reposition the sheets higher up as well. Run it once without dishes to give a clean.