As it is, this PrC looks to be way over the top. Several of the Dark Gifts look to be far too powerful.
Spell Immunities, Unholy Blessing, and Spell Resistance are particularly potent.
Suggestions for improving these:
Unholy Blessing - Split this up into two abilities, one for the bonus to saves, one for the bonus to AC.
Spell Resistance - Change this to read: "The [insert new PrC name] gains SR equal to 10 + 2 for every [insert new PrC name] level he or she has." As is, you are making the PrC completely immune to spells cast by an equal CR critter (as was pointed out, at 12th level he'll have SR 34, which a 12th level caster cannot get through).
Spell Immunities - Ditch this, there really isn't a way to make this balanced.