That speculation seems to be wholly based on the Nexus clip, which looks like it might show a Fel Orc along with what should otherwise be assumed to be the ex-Kirin Tor who joined the Blue Dragonflight (and who understandably use anti-Mage Felstalkers). It doesn't make much sense lore-wise for the Blue Dragons to ally with the Legion.
That said, I'm sure you'll run into Legion forces here and there, it just won't be a major part of the plot.
Actually, looking back over the video, it's clear that it depicts a human with brown hair rather than a Fel Orc. See the SS
here. I'm going to edit the Wowwiki page.