So the undead trees (once grown) have nifty blood drop particle effects, but they never leave any blood stains on the ground. I thought these were meant to be used for automated blood collection. Is this working as designed, a bug, or is there just a config option I didn't see that is perhaps disabled in the modpack I'm playing?

Those drops are purely cosmetic. You can shear the leaves and squeeze them for Blood using Integrated Dynamics or Forestry though.

Their main use is the crafting of Dark Sticks.

Oh, right. My bad.

So they're completely useless without a squeezer from another mod?