It's strange - in POB (and, I assume, in game) this effect stacks, but based on GGG's own wording conventions it shouldn't. Explicit "You have" and "you take" modifiers specifying a static value are not meant to stack or be modifiable. If I were reading these mods without knowing they stack I would assume that they don't. It should be worded "You take +20% of damage from blocked hits" rather than a static number.