I was considering taking the Envy domain because I happen to have the supernatural ability to drain ability scores like a succubus. Would the Envy domain's granted power apply to this supernatural ability? What about Spell Focus (enchantment) applying to some of the enchantment-school supernatural abilities?
Typically, effects that specify they affect only spells don't also affect supernatural abilities
Feats like Spell Focus (Player's Handbook 100), metamagic feats like Quicken Spell (PH 98), and domain granted powers that say they affect spells (like the granted powers of the domains Creation (Spell Compendium 272) and Envy (SpC 273-7)) typically affect only spells, not special abilities like extraordinary abilities, spell-like abilities, or supernatural abilities.
For example, a succubus (Monster Manual 47-8) that wants to improve its supernatural ability energy drain could take the feat Ability Focus (energy drain) (MM 303), but its supernatural ability energy drain wouldn't be improved if the succubus were to take the feat Spell Focus (necromancy). Similarly, an umber hulk's supernatural ability confusion gaze (MM 249) (which creates an effect like the spell confusion (PH 212), a spell of the enchantment school) isn't improved by the feat Spell Focus (enchantment).