Defiance of Destiny, works for blocked hits or not?

"Gain (25-35)% of Missing Unreserved Life before being Hit by an Enemy"

Blocked hits are still hits, they just deal no damage. The mod implies that I should be able to gain life before I block a hit, but the wiki reads "This modifier instantly recovers a percentage of your missing life before a hit deals damage to the wearer"

which one is true?

You can test this by toggling glancing blows. I don't know the answer, but wild speculation is that amulet won't work on hits that deal no damage, which include blocked hits without glancing blows or similar mods. If it worked, magnitude wouldn't have been whooping 35%.

Block requires a hit (i.e. you did not avoid the hit, therefore chance to block is rolled). That is why you can get stunned from hits that you block, and that's also why it's called stun & block recovery

So yes, Defiance of Destiny most definitely works with blocked hits