Cel, that particular Jiao-Di is one that carries the PB (pitch black) trait, hence the black swirls. When the Trait is carried as xX, it results in the pretty black swirling, and sometimes red eyes and a slightly more sinister looking Wyrm. The reason is that when bred to XX, the pitch black gene results in a Nightmare version of your Wyrm.....solid glittery black, glow in the dark eyes, and a very warped appearance. They are also sterile, being neither male nor female. We actually have three Nightmares now, two of which have been registered and one of which will be on their way to their new home very soon.
http://twilightdays.org/wyrmwiki/index.php/Mara
And it's twin http://twilightdays.org/wyrmwiki/index.php/Moroi
Aaaaaand their mum and dad are Asana and Twisted
So not only is it possible to get a Wyrm like that in a blind draw, you can breed specifically for it! Ask Shadra...she collects the evil little buggers! ;D
In short, two wyrms with xX pitch black can breed and create XX which is nightmare. So you've got it right. Breeding a wyrm without the gene at all to one who is xX on the other hand will not get you nightmares.
