Grants Def+6 during combat if unit initiates the attack.
These skills are great on offensive units who love initiating fights. Death Blow is the most popular, giving a potential 12 extra damage provided the unit attacks twice. Darting Blow helps units get follow-up attacks, and hits a crucial number: Six speed is enough for a unit to double attack a unit with the same Speed stat as itself. Armored Blow is useful for melee units who wish to avoid retaliation damage, and if the enemy would attack twice, they benefit from the defensive boost twice. Warding Blow is the least useful of the pack, since tome users cannot retaliate from melee, and often die in one hit to attacks from bows and daggers.
As they only take effect on the turn a unit initiates combat, they synergise best with offensive units. Death Blow is by far the most popular, with the mixed variant Swift Sparrow being second. Brave Weapon + Death Blow is an infamous combo, granting a guaranteed +12 damage right away. Swift Sparrow's Speed bonus makes it more popular for builds designed to quad-attack an opponent. The variants that grant defensive stats (such as Armored Blow and Warding Blow) are rarely seen as defensive units would rather run skills that work on the enemy phase.
As these skills require their user to initiate attacks, they can be averted by initiating attacks of your own. Death Blow, Brave Weapons and Desperation are some of the best counters to Blow skills in general, as they can help you KO foes before they get the chance to attack. Defensive skills such as Steady Breath, Warding Breath and Steady Stance can punish an attacking foe and mitigate some of the damage.