User blog comment:Jack Pistol/The First Dragonborn?/@comment-1992639-20121219223728

We know the Dragonborn ofcourse as the kind that can intuitively use dragon shouts and absorb dragon souls, the Dragonborn (or Dovahkiin). Alessia was mortal who was favoured and given power or privilage by Akatosh. We know that Alessia was given Akatosh's blood only when she was already of old age and in her deathbed.

Then there's Dragon-Born, someone who is born under the constellation of the dragon, which basically means someone born in Imperial territory. The Nerevarine is called as being dragon-born in Lost Prophecy but it is stated by Tod Howard that the Nerevarine was not the same sort of dragon-born as a Dovahkiin.

So I guess Alessia is only technically a Dragonborn. She had Akatosh's blood but by that time she was an old and frail woman. Miraak was the first Dragonborn to employ the Thu'um probably. It is possible that people also chose to forget Miraak, much preferring a more benevolent being such Alessia as the first Dragonborn.