All opponents are considered to have total concealment (50% miss chance) against the blinded character.
The subject of this spell ... benefits from a 50% miss chance as if it had total concealment.
Your target gains a 25% miss chance on all attack rolls.
So from what I'm reading and hypothesizing, if a target of Calamitous Flailing is blinded or attacking a concealed enemy, they would not only roll for concealment, but also an additional miss chance. Judging by the way the spells determine who the target is, these should stack.
I think they'd apply separately (effective miss chance 62.5%) rather than stacking directly for a 75% miss chance, but yes they'd work together.
Note that Calamitous Flailing is a curse, and it's 25% while like a miss chance is actually a cursed targeting choice. It has no overlap with miss chance. This is how you run it:
1) PC declares and rolls attack.
2) GM rolls Calamitous Flailing curse % to see who is really targeted. [1-75: Declared target. 76-100: Ally if appropriate, self if not.]
3) GM rolls blindness/whatever miss chance of the true target. [If an enemy is displaced, but curse triggered, the enemy's defense does not apply, so no miss chance to hit ally/self.]
4) If attack passes miss chance, check AC for a hit.
/cevah
Yeah, you roll separately for both instances, you don't add them together.
*To clarify either blinded or displacement would apply, but not both. The OP seems to be aware of this, just pointing it out for anyone else in the thread.