It's not exactly a well known art - in fact Riku doesn't know anyone beside himself who knows how to control Darkness the exact same way he does. He's tried to each the other Sora some of it, but well...

Still, he smiles at that confused look and then twists his hand to summons a tendril of darkness that spin it into a small sphere right above his palm before closing his hand and dismissing it again. Shion is far from the only one who ever had issues, though Riku's was never uncontrollable rage. No exactly anyways.

"When you feel ready, come to the clearing beyond the hedge maze on the twenty-ninth floor."