False Contrition states "Play during combat after your attack deals damage." The rules state that if an attack is successfully blocked the attack does 0 damage. I also know that there are some other attack cards that on their own do 0 damage so does that mean that I can use this card even if my attack is blocked, or not?

Also I was wondering if you can use multiple copies of "Superhero Blood Transfusion" in response to one attack decreasing the amount of damage by 6 instead of 3.