now i use cul a lot, shes my primary vocaloid and i like to make my own appends for fun so ive made a "black" append which is a more mature and soft voice for cul and a "red" append which is a more powerful and clearer voice for cul (it sounds a lot less nasal bc i xsy with megpoid power) and a falsetto which is cul xsy with kokone which sounds really cool :)
I think the issue with dark is its a normally one of the soft-expression types. This would make her sound very different since her original voicebank is a "solid" type.
Most updates seem to add a "soft" or "power" or both... So to me a "dark" append sadly isn't straight up the most logical step. And Internet co hasn't done a "dark" type as per say or a straight up one so it makes it less likely. :-/