As someone pointed out to me, even when the dealer does a pan drop to change the filter they typically do not drian the torque converter, which retains a lot of fluid. Therefore, when people take their cars to the dealer for this particular service, (or do it themselves) a substantial amount of the old fluid stays in the car and in a few miles of driving it mixes with the new fluid and turns it black. If you are not going to drian the torque converter, you may want to drain and replace the fluid once more shortly after a pan drop to try to get out as much of the dirty fluid as possible.