Blackberries are as much a part of California as creeks that go dry in the summer, T-shirts and jeans, or Knott's Berry Farm.
Though they ripen in the summer, they must be planted now, in the dead of winter, when they are available bare root. And it will probably take two full years for them to start bearing fruit.
While they may sprawl for 20 feet or more along a creek bank, in the garden they are easily trained on a trellis.
Blackberries and boysenberries are practically the same thing; Boysen is actually a variety of blackberry. So is Ollalie, though it, too, is often called an Ollalieberry. The other variety with a longstanding history in Southern California is the Young, with very sweet fruit and very thorny canes. Other varieties sometimes grown here include Cascade, Marion, Himalaya and Thornless Evergreen, but I would try to find Boysen or Ollalie.
Unfortunately, what you are going to find at many places selling bare-root plants right now are varieties not suited to Southern California. If you find a variety you haven't heard of, ask if it will grow here--but if it was packaged in the East, I'd be suspicious.
The support for blackberries should be sturdy. You can tie them to a fence or a wall of the house, or build a free-standing affair just in front of these structures--which is easier to tie to. Use a post-hole digger and put in two 8-foot-tall 4-by-4 posts so that about 2 feet of the posts is buried. Attach two wires to the posts, one 3 feet above the ground, the other 1 1/2 feet above that. Or, nail 2-by-4s where the wires would be. Posts can be 8 feet apart and one blackberry plant goes between each pair. Two plants are plenty.
Blackberries like good soil, but can survive just about anything. They do needs lots of water, which is why they grow beside creeks, and some fertilizer in spring and early summer. If they don't grow by leaps and bounds, water more often or more thoroughly.
Canes that grow the first year can be allowed to sprawl under the trellis until they are about 8 feet long. At that point, cut the tips and tie them to the trellis, weaving them around the two wires so they take up less space. These canes will go semidormant next winter and will produce fruit that summer, on little side branches that sprout from the main canes in the spring.
In the meantime, a new batch of canes will grow next spring. These can be left on the ground while the others ripen fruit. Then (don't lose me now), the canes that have borne fruit should be cut to the ground and the new canes tied in their place.
In other words, blackberries are biennials--growing canes one year, fruiting on those canes the next, while growing more for yet another year. You get bushels of fruit every summer, but the canes require two years of growth. Just be sure to prune off those that have fruited, usually in August, or your backyard may disappear.