Question: What 15 km river, located in southern Germany, whose name means "Blue Pot" flows east through Blaustein to the city of Ulm? 
Sentence (from "Blautopf"): The Blautopf (German for "Blue Pot"; "blau" means "blue", "Topf" means "pot") is a spring that serves as the source of the river Blau in the karst landscape on the Swabian Jura's southern edge, in Southern Germany.
Relevant evidence: Yes