Nebulous
Legend
Not in the FACE: On a critical hit the target must make a CON save (DC damage dealt) or be blinded for one round.
How about this - tying cool powers to "just" critical hits often means that the 5% chance of it happening just doesn't happen. 13th Age approached this by adding triggers to the d20 roll, such as rolling a Natural Even hit, or rolling 16+ on a hit, to trigger the secondary power. In this case, I would say "On a 16+ hit, you are Blinded if you fail the Con save equal to the damage." That offers a 25% of the condition, and on a Critical hit, that save will (likely) be impossible to make. Well, it really depends on how much damage is rolled, and that depends on the nature of the attack.
Not in the Face - On a hit of 16+, the target must make a CON save (DC damage dealt) or be blinded for one round. Alternatives: Use a Constitution save to resist a physical attacks, a Dexterity save to dodge an attack, a Strength save to push away an attack, an Intelligence save to resist a psychic attack.