Well, I don't know if I'm a real expert, but I'm sure the people who
wrote _Hortus Third_ qualify, so I'll paraphrase what they have to say.
The genus _Rubus_ has over 250 species, spread throughout the world.
Blackberries and dewberries differ "only in that dewberries are vines,
running over the ground, or needing support when planted." 38 species
are listed as having horticultural significance in North America.
_Rubus leucodermis_ (no common name) occurs from British Colombia to
California, east to Montana and Utah, with yellowish leaves and canes
and red or black berries. _Rubus macropetalus_ is a blackberry or
dewberry native from Northern California to British Colombia; it
has weak spines and comes up straight like a blackberry but then bends
over and vines like a dewberry. _Rubus nivalis_is a woody, prickly vine
which grows to 3 feet or more and is native to Idaho and Oregon north to
British Colombia. _Rubus pedatus_ is a nonwoody, slender, prickle-free
vine that grows from Alaska to Oregon. Salmonberry, _Rubus spectabilis_
, has a similar habit to _R. macropetalus_ (but with salmon-colored
berries) and grows from Idaho and California to Alaska.
My choice for a native blackberry to keep dogs out would be _Rubus
nivalis_. Where you can find them, I have no clue. I can send a more
detailed description of any of these, if you need it.