Unfortunately 50 turns is an arbitrary number, why not 40 or 60? What about different levels of CB difficulties (not same between normal and UNM: 50 is not much on the first, huge on the second).
I feel it would be better to have something like : CB will throughout ignore unkillable and block dmg 50% or 25% of the time.
Or something like: CB has 50% chances to remove unkillable/block dmg on each enemy on one or both of his aoe.
This will allow "regular" comps to use them (for instance blind seer has a block dmg to protect allies just enough time that they have a chance to hit cb once and hopefully get healed by lifesteal), while it will be enough to progressively break those teams who exclusively rely on these debuffs and nothing else (def, hp, etc.).
The pb I see for "regular" teams is that they have enough hp, def to resist until the 50th turn on nm CB anyway, they only need these occasionally after that (like in the blind seer example above)...
Note: I do use Blind Seer, and my team can survive about 60 turns on nm without ever relying on her block dmg: I rely on def + hp until 60, after that I can only use her A3 once to revive 1-3 allies hoping that their GS will proc on most before CB attacks again. There's already a rng aspect to it, my above proposition will not change that fact.
