Here's the latest version, ultimately dropping the idea in my previous post...
Blessed
A blessed weapon is good-aligned, meaning it can bypass the damage reduction of certain creatures. Up to 5 points of damage reduction can be bypassed with this weapon. In addition, all critical hit rolls against evil-aligned foes automatically succeed, so every threat is a critical hit. This last effect does not stack with any magical effects related to critical hits (such as flaming burst, keen, and vorpal). It bestows one negative level on any evil creature attempting to wield it. The negative level remains as long as the weapon is in hand and disappears when the weapon is no longer wielded. This negative level never results in actual level loss, but it cannot be overcome in any way (including restoration spells) while the weapon is wielded. Bows, crossbows, and slings so crafted bestow the blessed power upon their ammunition.
Faint transmutation; CL 5th; Craft Magic Arms and Armor, bless; Price: +1 bonus.
Defiled
A defiled weapon is evil-aligned, meaning it can bypass the damage reduction of certain creatures. Up to 5 points of damage reduction can be bypassed with this weapon. In addition, all critical hit rolls against good-aligned foes automatically succeed, so every threat is a critical hit. This last effect does not stack with any magical effects related to critical hits (such as flaming burst, keen, and vorpal). It bestows one negative level on any good creature attempting to wield it. The negative level remains as long as the weapon is in hand and disappears when the weapon is no longer wielded. This negative level never results in actual level loss, but it cannot be overcome in any way (including restoration spells) while the weapon is wielded. Bows, crossbows, and slings so crafted bestow the defiled power upon their ammunition.
Faint transmutation; CL 5th; Craft Magic Arms and Armor, bane; Price: +1 bonus.
...and here's a couple variants thereof, inspired by Darklion on the WotC boards...
Blessed v2
A blessed weapon is mildly good-aligned. Though this does not allow it to bypass the damage reduction of evil-aligned creatures, the weapon deals an extra 1d6 points of damage against all of evil alignment. In addition, all critical hit rolls against evil-aligned foes automatically succeed, so every threat is a critical hit. This last effect does not stack with any magical effects related to critical hits (such as flaming burst, keen, and vorpal). It bestows one negative level on any evil creature attempting to wield it. The negative level remains as long as the weapon is in hand and disappears when the weapon is no longer wielded. This negative level never results in actual level loss, but it cannot be overcome in any way (including restoration spells) while the weapon is wielded. Bows, crossbows, and slings so crafted bestow the blessed power upon their ammunition.
Faint transmutation; CL 5th; Craft Magic Arms and Armor, bless; Price: +1 bonus.
Defiled v2
A defiled weapon is mildly evil-aligned. Though this does not allow it to bypass the damage reduction of good-aligned creatures, the weapon deals an extra 1d6 points of damage against all of good alignment. In addition, all critical hit rolls against good-aligned foes automatically succeed, so every threat is a critical hit. This last effect does not stack with any magical effects related to critical hits (such as flaming burst, keen, and vorpal). It bestows one negative level on any good creature attempting to wield it. The negative level remains as long as the weapon is in hand and disappears when the weapon is no longer wielded. This negative level never results in actual level loss, but it cannot be overcome in any way (including restoration spells) while the weapon is wielded. Bows, crossbows, and slings so crafted bestow the defiled power upon their ammunition.
Faint transmutation; CL 5th; Craft Magic Arms and Armor, bane; Price: +1 bonus.