So the obvious question is: what can a Dark Apostate do better/different from a regular cleric? And I think the answer is: not a lot, unfortunately.
One option is Death-domain cleric using a mix of Divine Disciple and Dark Apostate trees. The DA damage options are weak, although no-save debuffs are nice; and Shadow form means you're healed every time you spam Mass Inflict on nearby enemies. I think DiDi is still the stronger caster tree, though. Use Magister as your primary ED for Necromantic bonuses and Time Stop.
The other viable option IMO are battleclerics. Apostate's Curse is one of the better imbues, its main downside is it only works with Favored weapons. Axel has a DA battlecleric thread; you could also use Dragonlord 5 Dark Apostate 15 with T5 Dragonlord and T4 Dark Apostate as a alternate version of the usual battlecleric options. Pick a deity with a strong Favored Weapon; or go for one of the racial options (e.g. wood elf Faith of the Forest for falchions).
Both concepts are still fairly niche alternatives to regular cleric, but they're the only ones which actually try to make use of DA's unique features IMO and not just treat it like something to be endured for past life feats.