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It wasn’t a black hole at all. It was a COMPLETE KAMUI performed by an Uchiha with both eyes. Every kamui has a black epicenter that becomes whatever is absorbed, which is why her attack is covered completely in iron sand. Obito and kakashi were able to target themselves, the opponent, or an object, in this case, Sarada targeted every grain of iron sand. A black hole would have taken in mass objects, not just one specific thing. Sarada simultaneously dropped a kunai that would have also went to the sky if this were gravity based. Uchiha experts such as myself recognize that to be a set up for an izanami. That tells me her eyes are completely different. She may have actually awakened an Eternal mangekyou.