Spells do not have a chance to hit (spell always hit and does not take evasion and accuracy into calculation) , the ability to hit with a spell is unaffected by being blinded. Thus, using this support against monster that use spell did not have any effect. Some unique items still provide additional damage against blinded monsters.
An attacker's chance to hit with attack skill, is the direct opposite of the defender's chance to evade. Since player characters with Unwavering Stance cannot evade, attacks made against them always hit. As a result, even blinded monster will always have 100% chance to hit a player character with Unwavering Stance (if assuming zero dodge), since the Unwavering Stance character always has 0% chance to evade. Similarly, attacks made by characters with Resolute Technique cannot be evaded, so blind has no effect beyond the sight radius reduction. This also applies to attack that have the mod Hits can't be Evaded during PVP.
Blind Support cannot support Flesh and Stone, since the blind debuff from that skill is coded as an aura.