I am pretty certain only this is imaginary, and the usage of the blacked-out page here was to indicate what was happening in her mind (namely, that she lost it in that moment), with everything that followed afterwards being real.
But yeah, this was basically violence that just happened to land on someone who was not upset later, not an actual BDSM play. It does subtly (or not-so-subtly, depending on how you look at it) convey that the attraction is mutual here, so there is that. Makabe being into the other girl was pretty straight-forward, but Hoshizaki was a tougher nut to crack, what with all that boyfriend talk and not really showing anything resembling attraction towards Makabe until she got hit.
Hoshizaki deliberately tried to get a rise out of Makabe, talked to her about being interested in pain play, and once Makabe snapped and started being violent, Hoshizaki reacted with barely any surprise, let alone negativity, almost like she expected and/or wanted it. And those violent scenes look pretty blatantly sexually charged. I mean, if a friend slapped me hard across my face for pulling what amounts to a minor prank on them, literally the only situation where I would not be upset was if I was heavily into said friend, if I was into pain, and if I was extremely forgiving about the circumstances (you know, the whole this-is-not-actually-BDSM-this-is-just-violence thing).