I went on an evening cruise up the Danube. It was so beautiful.
YESSS. Those places are beautiful for a reason. The Danube isn't blue but most water sights reflect the sky anyway. Gray sky = gray sea/river/lake. The BLUE Danube must have been really blue on a sunny, summer afternoon.
Wikipedia: The name of the river originates from the a mythological source, from a Latin Roman river god, known as Danubius, or Danuvius. Additionally, the meaning can actually be derived from the ancient indigenous Slavonic tribes who called it the Great Water.
Further down the course, where it meets the Black Sea it is known as Istros, which is also based on a mythological river god, as he was known in Ancient Greece.
"The largest of the rivers of Europe, the Istros, rises from only a few springs and moves in a direction facing the first assaults of the sun. Later, many Rivers rise with one accord as though they were escorting him--for he is the King of the Rivers of that country--and flow perpetually, and those who live on their banks know each one by name. But as soon as they discharge into the Istros, the name which they had at their birth ceases to be used, they surrender it in his favour, all are called after him, and together pour their waters into the Euxeinos". - Aelian, Greek natural history C2nd to 3rd A.D.