I blacked out the other person and sent him a message with nothing but a red exclamation point. Did the other party hack me, too?

If you blacked out the other party yourself, of course, there is no way to send him a message, because the other party can't receive it, and the other party can't send you a message. If you drag it back to send him a message or display a red exclamation point, it is that he deleted you or blacked you out. Maybe he thought he was hacked and didn't want to get back together with you, so he deleted you.