I know "ins Blaue" mostly from another context:
Der Schuss ging ins Blaue.
Here, it means that the shot went anywhere but where it was supposed to be going ("in den Himmel", figuratively)
This has since been adapted to (metaphoric) use, e.g.
Ich habe einfach ins Blaue hineingeraten.
meaning you just guessed randomly.
So, in this instance, the sentence means something like
.. doch das war nicht der Gandalf, der dafür verantwortlich war, dass so viele brave junge Leute auf gut Glück in die Welt zogen, um verrückte Abenteuer zu erleben."
You may want to check the original text for verification (I don't have it handy).