I have always believed that King's blood means a connection to the targaryen line. And the greatest acts of magic worked to date in the series (dany and jon, assuming he's also resurrected in book 6) seem to reflect that.
Magic is real in that world and when one is looking for 'realism' one has to acknowledge that. So things like blood magic and the power and presence of king's blood are likely to have deeper layers.
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