The Repairer of Reputations
Blessed weapons fare better against undead and unholy creatures
Blessed weapons hurt undead and unholy creatures – such as summoned undead, spirits and demons. They do full damage against ethereal undead (wraiths, ghosts, banshees and etc.)
Blessed items have a weak Dominion/Divine aura and draws the irk of creatures who desist such auras – the fey, undead and such. Spellcasters’ have such a strong magic aura that it negates a blessed weapon.
A mundane blessed weapon will have it blessing dissipate within a day. There’s no formulaic way to create a permanent blessed weapon.