This painting had several tears in the canvas including a large tear with paint loss where the canvas was folded out of plane making the tear appear as a large hole. The painting surface was covered by a discolored varnish, a thick grime layer and old retouching.
The dark spot on the cloud in the upper left is old discolored oil based retouching. Retouching with oil paint tends to darken with time and can be difficult to remove. For these reasons resin based “restoration” paints should be used for retouching.
The largest area to be retouched was the repaired tear. Note how the cloud in the upper left appeared without the darkened oil overpaint.