Indeed. Some additional effects are nulled if you take 0 damage, some others go through. The same happens with knockbacks: Twintania's divebombs will cause no knockback if you take no damage (either through HG or a buffed shield like Adloquium), but the first and second bosses in the Vault will deal knockback even if you reduce the damage to zero. There's really no way to tell until you try it, so don't be afraid to experiment, etc.

Also, yes, for the most part, you're better off not using Dark Arts on Power Slash. HOWEVER, I don't think we have numbers about the enmity modifiers for Stormblood. The new Job Guides state that the enmity effect on Power Slash has been increased, but it doesn't specify if only the base effect has increased, or if the Dark Arts bonus has also increased. We'll have to wait for the math wizards to dig that info up .