When you bless someone and say God bless you, you are giving a spark of your divinity to the person. You are conveying your happiness. In that moment you are passing that positive energy to him/her. You never know how that blessing will work wonders for that person.
Blessings are not only when given in the state of happiness. They can be given even to the person who has hurt you, given you pain, cheated you.
When you say God bless you, you are conveying that you have forgiven already. That you are leaving the decision in god’s hand about how he wants to bless you.
If you are right, if he feels whatever you have done is ok, his blessings will be showered the way he wants. If not, then perhaps it was your karma which you had to go through.
So carry no grievances against the person who has hurt you. In fact, free yourself from further karma by freeing yourself from him by saying GOD BLESS YOU!