Yeah, the first time he did it black was when he was in Bankai. Not Hollow. Even though it was obvious he was drawing on his Hollow power to do it. I think the writers got confused.

And no, Renji had never seen him use it before.

Renji has however, experienced the white one first-hand.

He's probably like, 'Hey, that's that attack that almost cut me in half that one time, only now it's black."