This only appears once, in Tolkien's notes in the discussion of Kheled-zâram. He mentions Lake Hele(ð)vorn, meaning "Black Glass" in Sindarin, and says that this is probably a translation of a Dwarf name such as Narag-zâram. He goes on to say that the root N-R-G is "black", as evidenced by the Dwarves' name for Mordor: Nargûn. The way the passage reads, it sounds as though Narag-zâram literally means "Black Glass", but this is clearly not the case, as he states several other places that Sindarin heleð "glass" is taken from Khuzdul kheled. From that, zâram can't mean "glass", so the paragraph appears to be written in an odd manner or else Narag-zâram isn't a literal translation of "Black Glass". (Or, quite possibly, this author is simply missing something!)
On Parma Eldalamberon XVII: Words, Phrases and Passages, pg 37, Tolkien gives a possible Dwarvish origin of the river Narog's name, which is Narâg. The other places we see the template CaCâC are in plural nouns, such as Khazâd. That could mean that the name refers to many layers of "blacknesses". Semitic adjectives usually agree in number with the nouns they describe, so this could be another example of a shortened nick-name in Khuzdul. The missing name might be something like "waters". Another possibility is that adjectives simply use this vocalization as a singular form, unlike nouns. This seems more likely, as we don't see any adjectives that have the template CuCC, as in Khuzd. This would allow the full name of the Narog to simply be "Black River" in Khuzdul: perhaps something like Narag-nâla, but shortened to Narâg.
Narâg is probably "black" and singular, nominative, indefinite.
Narag is "black" and is singular, nominative, composition.
Zâram is "lake, pool" and singular, nominative, indefinite, just like Kheled-zâram above.
Narag-zâram is "Black Lake" and is a compound word with adjective-noun word order.
The Return of the Shadow, pg 466
Parma Eldalamberon XVII: Words, Phrases and Passages, pg 37