From the photos, it looks to me like you've had moisture in the area, and together with dirt/carbon buildup caused some current to flow between the traces due to voltage differential. That resulted in the area heating up and turning black. If allowed to continue the traces on the circuit board would have opened up and eventually the board would have failed. I've seen it many times in industrial systems. Since you were able to clean the area and remove the discoloration, unless it caused some secondary damage you caught it in time. When I have to repair a board with that problem, once repaired I use a conformal coating spray to keep moisture and dirt away. Moisture loves bare copper, it turns it a nice shade of green.