Am I missing something here, or can Inquisitors cast spells of an opposed alignment? The rules for inquisitors say nothing about whether or not they can cast diametrically opposed alignment spells, which generally means, "go for it."
The problem with this is, what if a neutral good inquisitor casts Blasphemy... would it affect him as well? I know the inquisitor is meant to do the church's dirty work, but, is this just a bug in the playtest, or are they really meant to be able to cast diametrically opposed spells?
A good character that casts [evil] spells will drift toward a neutral alignment (and eventually evil) it they don't balance it out with good acts, so there is a long term risk of alignment drift, but nothing worse than for a good wizard.
Non-evil inquisitors probably don't want to cast Blasphemy, as it would affect them. An Inquisitor 13/Fighter 1 could safely cast it, as his hit dice (14) exceed his caster level (13). Or maybe he's being swarmed by low level mooks and willing to risk making the save.
I'm not familiar with what is on the inquisitor's list, but some evil spells won't have negative effects. Did the enemy caster just use Summon Monster to summon a celestial something with great natural attacks? Protection from Good sounds like the evil spell you want. Contagion could also have its uses.