If i remember correctly, all Black Dragons were consumed, or enslaved by the Tsaesci? And didn't make an appearance in Alduin's army during the game, whereas Red Dragons did (who were also enslaved by the Tsaesci).
When i saw the trailer, this http://www2.picturepush.com/photo/a/11472345/img/11472345.png made me think of the Black Dragons straight away... it also made me think of Fell Beast's from LOTR..

What do you guys think? It seems to be in Herma-Mora's oblivion which could explain where they've been all this time? Herma-Mora could have enslaved the Black Dragons for himself within Apochrypha, and we have to free them?

What do you guys think?

I really doubt even a daedric lord could enslave an entire sect of dragons and keep them under control
way too much work
maybe this one in the trailer got caught or willingly went to him
it doesnt really seem like a normal dragon though
it has more evil aquatic features like mora tends to have
to me it looks eelish
weird slender head, fins instead of spines or plates, really smooth skin

Well to me the dragon kind o looks purple like the dragon you can summon so i guess purple dragons are friendlies, but later in the trailer i think you see the dragon born riding the basic dragon.

Yeah you do, theres also a screenshot of him riding a Frost Dragon,
Im guessing any kind (except Parthunaax, Ohdaviing and Durnehviir) will be mountable.. lets hope anyway, a legendary Dragonborn needs a Legendary Dragon

He's probably another important dragon character. Although, it'll get a bit silly if we can summon this one (then ride it) too. Actually I wonder if we can ride Odaviing or the undead dragon.

I really dont like the look of that dragon, though its awesome that we have some diversity.

I really want them to release a DLC that just gives us a ton of different dragon types.