I would like to know why it is not mentioned in the spell of black tentacles that the damages are of the bludgeoning type when the tentacles succeed in grappling a foe as in rules 3.5? The question is to know if the reduction of the damages (Dr) works or not against this spell.
Sorry in advance for my bad English. ^^
The spell just deals (untyped) magic damage. No damage reduction will help you against that, you'll have to make sure they won't catch you.
What KaeYoss said -- DR is useless against tentacles of that magnitude!