I did exactly what you are describing 3 years ago. I knew less then than I know now, and basically I just took a shovel and dug up several wild blackberries with balls of dirt and roots all together. I took the plant and a rootball that was about as large as I could fit into a 5 gallon bucket. Most of the wild blackberry plants did survive and came out in the spring. (I dug up the plants in the winter). What surprised me is that it took 2 full years for the new plants to really thrive. The first year- at least from what I could see above ground- they basically just held their own and each plant only had one or 2 stems. Even the second year, they didn’t send up much in the way of new growths. They did have a few more sprouts that turned into long stems, but all within inches of the original plant. But then came the third year…and the difference was dramatic. I had wild black berry plants coming up as far as 4 feet from the ones I started. And LOTS of them. I kept the new plants to a limited area by mowing down those that came up too far away, but if I had wanted to just create a large, wild blackberry patch I am certain it would have spread as far as I would let it.
Now, all that being said, just because I can do something doesn’t mean I should. In my case, even in the 3rd year when new plants were shooting up everywhere and the older plants (“mother plants”) were very large, they never have produced berries anywhere near as large or as tasty as the patch I took them from. And- even the largest ones- produced very, very few berries. And I did fertilize and take care of them. During the exact same time period I planted several varieties of cultivated blackberries I bought. By the 3rd year they were producing 10 times as many berries, had much bigger and better vines, and the berries were almost as good as the wild ones (maybe as good in some cases). In short, for me it was a fun experiment, but a fools folly. I’d have been much better off to have just gone with commercially developed plants.
Please keep in mind that nothing in my story-including the way I transplanted- is being offered as the “right way” to do it. I didn’t know about root cuttings or other, better ways to do it. I also like in Tennessee, which is obviously very different from Northern CA. So none of this may apply or be helpful. But I thought you should know that even a beginner like me was able to transplant wild blackberries, but in the end it wasn’t even close to being worth it (for me, in my environment, with my wild plants, etc). But its fun trying things like this, so I wish you luck if you try it.
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