Welcome to the game!

BoC is "evil magic damage", whatever the hell that is, but not dark magic, so does not synergize with Soul Collector.

Dark Reflection is on cooldown when you get hit, presumably from melee-only.

Vampirism amplifies "all magical healing", which includes Torture Mastery.

Power of Sacrifice is a high-risk, high-reward spell (in PvP), but a low/moderate risk, high reward DoT/heal in PvE. Since Power of Sacrifice does 0 upfront damage, you can cast it on multiple low-HP enemies like mummy spiders to heal a great deal (where Curse of Pain may otherwise kill them). In my opinion, the damage is far too great to use on yourself or an ally (36 damage over 12 sec without Curse Mastery, 45 damage over 15 sec with Curse Mastery, ouch!), for just ~15% bonus damage (on average).

One notable exception: Cleric's with Perseverance perk, reduces damage, so they only take 1 damage per second, so you can Curse them to power them up without a massive cost.

Warlock