Zun mentioned for EOSD that Rumia is constantly enveloped for darkness but chose not to do it for gameplay/other reasons but said something like, "Please imagine she is covered in darkness."
Probably unable to generate them in the first place. Reimus and Marisas have nothing of the abilities from their real counterparts, so makes sense the others wouldn't either.
I thought the accepted answer was that they are not normally able to generate the darkness field, unless you try to steal its accessory; in which case, KYAG.
PMiSS gives ample explanation, Rumia hates the heat from the sun, hence the darkness, but it was night in EoSD.
Yukkuri Rumias are largely unused. The wiki has some stuff about them being ineffective predators, but that's often just one guy's ideas when it comes to rarely used types. I say they only use it to sneak or hide.
Rumia types as ambush predators? Creates darkness briefly to panic prey and in the seconds after letting the effect end, pounces. That's how I'd write them.
Ha, "accessory abuse." The image of a yukkuri shoving it's ribbon where the sun don't shine is both scary and hilarious.