hi gaz, i tried dark last year and found that my ybs didnt want to fly, i know some say this doesnt happen but ive also hear fanciers say the same as me and theirs dont want to fly either..............so i suppose dark works differently for different fanciers.
also found that mine got a touch of yb sickness which i cured fairly easily, i cant say for certain this was caused by having them on dark but ive heard it does, and it doesnt from different people.