This phenomenon is well known from the transit of Venus in front of the disc of the sun. This effect appears when the objects are not exactly focused (see: The black drop effect is not an atmospheric phenomenon). On Oct. 13th, 2013 I photographed this effect under unexpected circumstances:
The sun had just set behind the skyline of the Palatinate Hills across the Rhine valley, when a very bright contrail due to forward scattering of sunlight raised from behind the Hills. One of the hills covered the contrail, and the brightness contrast showed the drop phenomenon very nicely: the slope of the hill appears almost vertical where it is intersected by the contrail. Due to the far distance, I used the 13x zoom of the Canon Powershot A510. As the optics of such a small camera is limited, it provided the defocusing needed to show the effect. The sequence show the raising of the contrail during a time lapse of 4 1/2 minutes just after sunset.
If you may ask now: where is the “black drop”?: The “black drop” is somewhat hidden: it is the interface area between the bright contrail and the dark silhouette of the hill, where the “drop effect” raises the skyline showing an almost vertical slope of the hill in front of the contrail. The “drop” is best seen on the second and third frames from the bottom.
Author: Christoph Gerber, Heidelberg
Another article to this topic: The black drop effect is not an atmospheric phenomenon