I had the same issue when channeling Rapid Fire as a Demon Hunter. There seems to be two things in play here:
- It deals its damage per hit. For channeled spells, this is extremely fast.
- It scales up for harder difficulties. 20 damage per hit on Inferno would mean nothing. (sorry, no exact numbers)
This curse is applicable at all levels, it's in game from Normal onwards. The simplest way I found to deal with it is either:
- Don't get hit by it. The balls they fire are very slow moving and turn very slowly. This usually makes them fairly easy to dodge. If you have it, Teleport should make this trivial.
- If you do get hit, avoid channeled spells. They will kill you very quickly. Any other types of spell seem to do minimal damage in Normal and Nightmare (I haven't played Act 3 past there personally).
With respect to your last two questions about armor/resists, it's hard to tell at this point in time. There's very little information available about enemy abilities like this, and the lack of a combat log of any type makes it difficult to determine damage types. From my personal experience I don't think that using armor/resists to mitigate the damage coming from channeling is the way to go, just because of the sheer volume of damage that is done.
Blood Star related bug fixes in patch 1.0.3
Link to 1.0.3 Patch Notes.
- The Succubus "Blood Star" debuff will no longer affect the player if the damage from the projectile is dodged/blocked/etc
- The damage from the Succubus “Blood Star” debuff will now scale based on the current cost of a player's skill, even if the cost is reduced by items or skill runes