A Reign of Toads
King’s Blood is the literal blood of royals. Many believe it to have powerful properties, especially where blood magic is concerned.
Most people do not believe in the divine right of Kings. The Starks were Kings in the North by dint of good stewardship and military might. The same for the Lannisters, Gardeners, Martells, and Durrandons. But the Targaryens are different. They are the blood of Old Valyria and many think that the power of their birthright is lies in some preternatural power. As a result, King’s Blood almost always refers to a Targaryen (or a Baratheon, Blackfyre, or Velaryon) in Westeros at least.
The belief is somewhat stronger in Essos where blood magic often draws in the Blood of a Khal, Archon, Prince, or Magister. It is said that the blood of emperors is prized in YiTi and the lands around the Jade sea.
Some hedge wizards insist that the blood must be specially treated: kept in ensorceled bottles, mixed with arcane herbs or some other step.