SuStel:

If you're calling demons qa' Hurgh, why aren't angels qa' wov?

I will write the reply in DIvI' Hol, because I'm replying in order to
explain my choice of klingon words.

The "dark" as an adjective describing the demons, hasn't anything to
do with "evil". "dark" expresses the hidden/secret nature of these
beings, let alone that their appearance is far from "dark" "black" or
"scary". furthermore demons are quite capable of appearing as beings
of light too.

If I described the angels as {qa' wov}, I would be attributing to them
a demonic attribute (appearance-wise), and even worst I would be
describing them indirectly as being "good/loving/innocent", which they
sure as hell aren't.

You're assuming that Klingons use the same metaphor dark to refer to being hidden or secret. I see no evidence of such a metaphor in the canon. Frankly, I thought you were literally referring to demons as lacking in light.

Without a known metaphor, be literal. If demons are hidden, use qa' leghbe'lu'bogh unseen spirit. If they are secret, use qa' pegh secret spirit. Or find some other, more appropriate descriptor. But Hurgh just tells me that either they do not emit or reflect visible-wavelength photons, or else they're spirit-pickles.

-- 
SuStel
http://trimboli.name