On the topic of banebloodEdit

Humanity in the Infinite Sea is best seen as a pyramid. At the bottom sits 99.5% of the human race: those lacking the ability to sense the Bane within all living and formerly-living things. This inability marks them as "Baneless". The remaining 0.5% who do possess banesense are known as "Banebloods". Of these, perhaps one in one hundred are able to manipulate the Bane and bend it to their own will. They have the capability of influencing the objects and living things in which the Bane resides with the aid of material components like baneseals. These gifted few are known as Banecasters. While Banecasters may be born from the union of any two Banebloods, the child of a Baneblooded parent and a Baneless parent will not possess the Banesense, nor will any of their descendants. These offspring rare referred to as "Deathborn". 

In the Northern Kingdoms  (including Tierra and Antar ), one of the most important social distinctions is that of Baneblood: Only Banebloods may inherit noble titles , rule as monarchs or become knights of any of the religious orders . This means that in the Northern Kingdoms , the term "Baneblood" is almost synonymous with "hereditary nobility". While there are Banebloods without titles, they are still part of the aristocracy, a social class which no Baneless person may enter. In the interests of protecting both their noble blood and their pool of Banebloods, every single one of the Northern Kingdoms maintains laws that prevent Banebloods from marrying or having intimate relations with any one else save other Banebloods.Harsh punishments, up to and inclusing summary execution, are used to enforce these laws.

Regarding the loss of baneblood throughout centuries:

Baneblood used to be universal. Something changed that very suddenly. That's actually why Calligia (prodecesser of the League of Antar + Empire of Octovia) fell. That actually ties into the "Celestial Mandate": in the minds of the people of the time, this was clearly a sign that only those who had retained baneblood had the right to rule. The rest of the population had been perceived as losing that privilege through some sort of deficiency on their part. That's the interpretation in the Northern Kingdoms anyways. The Kian (naturally) blame the Takarans. The Mhidiyossi merely believed that it was the first step of the dying of the world, which led to an increasing trend of religious extremism which partially led to their current civil war. The loss of Baneblood is related to, but not a result/cause of the Curtain of Storms. The Curtain of Storms has always been there (though it has been receding slowly over the past few centuries, at the rate of a twenty or thirty kilometres a century.