Black Kyurem/White Kyurem cannot be captured because Ghetsis's "staff" when you first encounter it but when you recieve N's Zekrom (Black 2)/Reshiram (White 2), you can come back to the Giant Chasm and battle it again. When you come back, it will be at Lv. 70 in it's normal form and to become Black Kyurem/White Kyurem, it will need to be combined with Zekrom/Reshiram by using an item called the DNA Link Pin.