A fourth game in the Das Schwarze Auge series called Armalion was almost released, until publisher Ikarion had to quit business due to financial difficulties. The source code may have been released under another name, maybe even as freeware (that is what the Ikarion guys told me). A link with a gameplay trailer can be found in the related links section.
Publisher Topware used Schatten über Riva to conduct an experiment: the game entered the stores as a jewel case version, without a box and a regular manual. However, it only cost 50 Marks (rougly $25) -- about half the price of normal games. Topware wanted to see if a (fairly popular) game would sell more units if it was priced considerably lower than other products. The result was mediocre, and Topware soon released a box version follow-up.
Did you know that the producer of this Trilogy is Guido Henkel, also known as Guy Henkel, the designer of 1999 RPG of the year "Planescape-Torment"?