Ive never seen a non gold Darkwing. Ever. I am very hesitant to agree they exist and if they do it would 110% of come from the preview events like the inscr Purple Crystallines.
As for farming req8 Ikki is right in that farming levels 17 - 21 is your best bet, however the cap does go as high as 23, but youll be hitting the req ceiling quite a bit (Drops will role higher). Chest running is always your best bet for OS req8's, HM is your best bet for blue/purple req8 inscr, and Kournan chests and level 20-21 enemies in NF and EoTN are best for gold Inscr req8 I tried out farming Chromatic Drakes for Darkwings and that worked pretty well outside Central Transfer Chamber, but in NM the drop rate is just stupid low.
Kournen Chests drop so many req8 its crazy, get a few every 100 chests or so. Of course they are mainly focuses but heck its still req8!