Sorry, I was wrong about that. I couldn't find a single instance of me completely blocking one of his attacks against a target under 50% hp while DH was available. There was one potential... but then I realized that I had about 4 hp more than 50%. So I couldn't find anything to confirm or debunk whether it occurs with other champions' AAs or abilities.
I also tried two other games against opponents with DH, with no luck. Always either above the threshold, or the attack was blocked but DH was on CD and so couldn't trigger anyway.
Wanna test it out? We can just create a custom and test it out real quick by having you poke me down to 40% or so and then attack my shield with something that doesn't break it.