I think the question of "Which came first, the defense or the block?" is a bit ambiguous. First of all, I'll assume that we're talking about spell blocks here, since they are cancelled when they are bypassed, while other blocks are not. That said, the wording of the rule does leave room for interpretations: "Any attack that bypasses the block cancels it out" (YS252). Given that, it could be argued that the flavor text is key here; that is, if the attack never reach the spell (because it hit that pile of boxes instead), did the spell actually get bypassed?
I see this as being adjudicated in one of three ways, by way of houserules to clarify this:
1) The block is always accounted for first. In this case, an attack that beats a spell block will shatter it, even if the target successfully defends.
2) The block is treated as a backup defense. The defender using the block only when it is needed, and with a high defense roll it is not needed; therefore the block would not be bypassed.
3) The flavor text of the action determines which 'defense' gets tested first. In this case, a 6 foot dome of energy might get hit even if the dodge was successful (you might be moving a foot to the side, but the dome is still in the way of the bullet), whereas a skin-tight entropy field might not (if you dodged enough that it wouldn't hit you, then it wouldn't hit the skin-tight field).
One argument against flatly applying #1 is the way enchanted item defenses are handled. You can decide to use their shifts as a block or as armor after you compare attack and defense rolls, and you only mark off a use if the defense is needed. This would favor the #2 interpretation.
