"'Iw bIQtIqDaq jIjaH."
Translation:I travel the river of blood.
Yes. It is an idiom and that is the standard translation of the Klingon sentence. leng would be a great way to translate the English sentence into Klingon and it is accepted as a possible answer on the exercise in that direction. "Go" is also accepted as a translation in exercises of this Klingon sentence.
It's not an idiom, it's a metaphor. An idiom is a saying where the words don't mean what they say (e.g., a "piece of cake" has nothing to do with cake); a metaphor is where you describe one thing as a stand-in for another thing. There's some overlap, but this sentence happens to mean exactly what it says, even if it's only a metaphor.
The reason jaH is being used where the English has travel is because Marc Okrand isn't quite so strict with literal translations as you'll find on this site, and Okrand translated this sentence (it comes from the episode "The Icarus Factor").