[Edi]

It shouldn't be preserved. The special unholy stuff that Ermor and Desert Tombs C'tis had in Dom2 can be done with restricted national holy spells, and those were generally meant to boost the powers of undead creatures in ways other than bless. Note that they also generally affected ALL undead, not just unholy undead (which are precious few in number).

The blessings are separate and need to be applicable through normal bless. Why would a god who is the god of a nation that uses undead nad considers them sacred refuse to bestow his blessings on sacred undead? And why would the priests who also consider the undead holy refuse to do that?