Board Thread:Lore Discussion/@comment-10561605-20130820164006/@comment-3186827-20131004162951

Nixeu wrote: The thing is, it's actually quite hard to know for certain if any of the Septims were Dragonborn, since there were no dragons around to absorb the souls of. Dragons are not relevant here. You dont need to absorb a dragon soul to prove you are a Dragonborn. Alessia couldn't absorb souls:

"Alessia didn't have the power to absorb dragon souls. Hers was a much more nuanced power: to dream of liberty and give it a name and on her deathbed make Covenant with the Aka-Tusk." - MK

Technically there are two types of "Dragonborn" - those that can absorb souls and those that can't, but both are blessed by Akasoth. Also see The Book of the Dragonborn.