The 3rd-level Clr spell spark of life [necro] (Spell Compendium 196-7) does all sorts of horrible things to an undead creature, making it vulnerable to many things normally affecting only the living like…
- critical damage, ability damage, nonlethal damage
- fatigue and exhaustion
- energy drain
- effects requiring a Fortitude saving throw
In addition to the affected undead creature needing to eat, breathe, and sleep for the spell's 1-round-per-caster-level duration, the spell says, "While it is under the effect of this spell, both negative energy (such as inflict spells) and positive energy (such as cure spells) heal damage to the undead creature, rather than damaging it" (196).
So, as a sort of a left-handed blessing, but this has its uses—especially for an undead creature that wishes to conceal its type.