Paizo staff have somewhat answered this:
Paizo Employee James Jacobs Creative Director Mar 27, 2010, 12:25 PM
7 people marked this as FAQ candidate. Staff response: no reply
required. 7 people marked this as a favorite. James Jacobs
When a spell mentions that a specific type of damage caused is
bludgeoning, piercing, or slashing, that DOES have to overcome a
creature's DR. Some spells create magic effects, while others use
magic to create physical effects; that's a major theme of conjuration
magic (and creation magic in particular).
If you hit an ooze with the Split ability with the appropriate type of
damage, be that from a spell or weapon, it will split.
And if you drop a spell that, say, does piercing damage on something
with damage reduction like 5/bludgeoning, that piercing damage will
get offset by the damage reduction.
Casting ice storm on a mix of zombies and skeletons would indeed be
complex. The zombies would reduce the damage taken from the
bludgeoning portion of the spell but take full damage from the cold,
while the skeletons would just ignore the cold damage entirely and
take full damage from the bludgeoning.
MOST spells don't inflict bludgeoning/piercing/slashing damage at all.
And most spells don't inflict multiple types of damage either.
Lightning bolt, for example, just causes electricity damage. It
bypasses DR entirely but not electricity resistance or electricity
immunity. And unless the spell description says so specifically,
bludgeoning/piercing/slashing damage it inflicts is not automatically
also treated as bypassing magic. Again; the damaging object is CREATED
by magic and PROPELLED by magic, but is not in and of itself magic.
A spell that conjures a flight of arrows that deals piercing damage
should be reduced by DR/bludgeoning or slashing. If it doesn't, then
that spell's damage type shouldn't be listed as piercing at all, but
untyped damage. Spells and effects that do untyped damage are pretty
rare in Pathfinder, since these spells are quite powerful since their
damage can't be stopped by any form of immunity, resistance, or damage
Sean K Reynolds Contributor Aug 21, 2012, 11:51 AM 18 people marked
this as a favorite. Sean K Reynolds
There's no point in a magical ability calling out whether it's B, P,
or S damage unless the intent is that DR/B, DR/P, or DR/S resists it.
If the magical ability wasn't affected by any kind of DR, the ability
would just say it deals damage, and not list a type of damage at all.
Because the magical ability lists a damage type, effects that block
that damage type apply. If it doesn't list a damage type, then the
"creature takes normal damage from spells, spell-like abilities, and
supernatural abilities" rule applies.
But Pathfinder writers removed the word "Bludgeoning" from the 3.5 spell description of Black Tentacles. NOTE that while they removed the B-word from the spell they added it to the Unarmed Damage rule (it wasn't there in 3.5).
So an argument could be made that Grappling is typed damage:
Damage: You can inflict damage to your target equal to your unarmed
strike, a natural attack, or an attack made with armor spikes or a
light or one-handed weapon. This damage can be either lethal or
All those attacks have typed damage, even Unarmed:
Unarmed Strike Damage: An unarmed strike from a Medium character deals
1d3 points of bludgeoning damage (plus your Strength modifier, as
normal). A Small character's unarmed strike deals 1d2 points of
bludgeoning damage, while a Large character's unarmed strike deals 1d4
points of bludgeoning damage. All damage from unarmed strikes is
nonlethal damage. Unarmed strikes count as light weapons (for purposes
of two-weapon attack penalties and so on).
Dealing Lethal Damage: You can specify that your unarmed strike will
deal lethal damage before you make your attack roll, but you take a –4
penalty on your attack roll. If you have the Improved Unarmed Strike
feat, you can deal lethal damage with an unarmed strike without taking
a penalty on the attack roll.
And the constrict ability supports this:
Constrict (Ex) A creature with this special attack can crush an
opponent, dealing bludgeoning damage, when it makes a successful
grapple check (in addition to any other effects caused by a successful
check, including additional damage). The amount of damage is given in
the creature's entry and is typically equal to the amount of damage
caused by the creature's melee attack.
I would say the tentacles constrict for sure. I'm not sure whether you can choose nonlethal damage though.