User blog comment:Jack Pistol/The First Dragonborn?/@comment-149.150.237.236-20121218145808/@comment-2040576-20121218152158

Interesting point. I think you may be right, and I may have just read it ambiguously, although on the Dragonborn page it does say that she is believed to be the first.

This also still does not answer my question of how Miraak then became Dragonborn? We know he learned the Thu'um through his service to Hermaeus Mora, but that doesn't mean he got the blood or soul from him.