I don't feel like doing the e-mail thank, W.S., but if you chance to wander back this way again, maybe this will help.
I checked out the (very cool) website Daividh
mentioned this evening:www.roisindubh.tripod.com
And found the possibility of "Duvessa" (original Irish form: "Dubheasa") for you. It has a meaning kinda close to the meaning of "Douglas
" ("blue-black"), and it's original Irish form is kinda close to that of Douglas/Dubhglas.
Oh, and Duvessa/Dubheasea means "dark lady of the waterfall".