Plane Sailing said:
It is very cheap (same price as keen, much better benefit!)
It's been a while since I did this kind of math, but I think I got it right. (?)
(I'm tired of thinking about it.

)
X = average damage, not crit, no extra damage-dice
Y = chance to hit
Assuming roll within critical threat range will hit.
Assuming 18-20/2x base crit
Regular on hit: 1,15X
Keen on hit: 1,3X
Bless on hit: X + 0,15X/Y
Bless/imp.cr. on hit: X + 0,3 X/Y
Keen/imp.cr. (not allowed in 3.5) on hit: 1,45X
Examples:
1) Scimitar, 12 damage on average hit (X=12). You hit 70% of the time.
Keen on hit: 15,3 damage
Bless on hit: 14,6 damage
Keen/imp.cr. (not allowed in 3.5) on hit: 17,4
Bless/imp.cr. on hit: 17,1
Keen is better.
2) Scimitar, 12 damage on average hit (X=12). You hit 30% of the time.
Keen on hit: 15,3 damage
Bless on hit: 16 damage
Keen/imp.cr. (not allowed in 3.5) on hit: 17,4 damage
Bless/imp.cr. on hit: 24 damage
Blessed is better
Conclusion:
Compared to
keen,
blessed is better if you're fighting an evil opponent
and have a less than 50% chance to hit. It get better compared to keen when you add Improved Critical - then you only need less than 67% chance to hit to benefit.
Blessed is fairly strong combined with Improved Critical, but not overpowered. At first glance it would be underpowered at +2. Evil creatures aren't
that common, and while the ability to penetrate DR/good is nice to have, it's not worth that much overall.