Can a choose the blocker ability choose a card in owner's hand or must the blocker be in play already?
Two examples would be king Amaziah and Obedience of Noah. My gut would say no because the text defaults to in play, but I could be wrong.
King Amaziah: "Once per game, you may choose an evil king of Israel to block..."
Obedience of Noah: " Holder may choose the evil character opponent uses to block"