Fire bless adds +4 armor piercing damage countered by fire resistance.
Death bless adds +2 armor negating damage countered by magic resistance.
Not sure about affictions caused by combat magic, manual tells bless gives bonus to it, but I never really paid attention, I usually want my enemy dead, not wounded.
I'm pretty sure units you would like to cause affictions to wouldn't be harmed by blade wind at all.
There was a test by Calahan proving death bless does not affect ritual damage spells.
Each atack decreases defender's defence by two.