So, I was playing Magic against my dad and he declared an attack with a multitude of creatures. I won't go into which ones since their effects and such are irrelevant to this; their attacks and current states in play weren't being affected by any sort of spell effect or counter.
I played the
Safe Passage 
instant from the M13 set in response to the declared attack since I have very few creatures to block with. Safe Passage reads as follows:
"Prevent all damage that would be dealt to you and creatures you control this turn."
After I played that, I still chose one creature to block one of his with because it was 1/1 and it would be blocking a 2/1, so the 2/1 would die. However, my creature would still be alive because I played Safe Passage, so any damage that would be dealt to it would be prevented. It was a no-lose situation for that attack/block scenario between those two creatures, or so I thought.
My dad's resulting argument was that blocking wouldn't matter at all, and as such should be totally skipped even though an attack was declared, because all damage that would be dealt to me is null and void thanks to Safe Passage; he thinks I shouldn't be able to block because I played that card.
My question is this: is my call correct, or is blocking after playing Safe Passage in response to an attack totally unallowed?
