The Blood King is not a title given to one man alone. Beginning three generations ago, the current Blood King was paranoid about assassination attempts and thought it best for the world to see one "king" while he, the true king, ruled from the shadows.
The second generation, Katsu's father, took this a step further. He wanted both his and his son's identities kept hidden so he had decoys made for both of them.
While it is not necessary for the decoy to be of royal or noble blood, he does have to be a skilled warrior. It is also necessary for the Blood King to trust his alternate. If a decoy dies than another will simply be chosen.