I live in the bay area and looking for the same. I am planning to go tomorrow or Monday. Any details on the specific area where you were that I can GPS?
Let me tell you, I picked that fairly extensive plot pretty damn clean. I'm pretty good at it now, having personally picked several gallons of wild blackberries over the years.
I'm confident that there are many many other patches with similar amounts in Tilden Park, in the Oakland hills, along Wild Cat Canyon Road in the Berkeley/Oakland hills.
I'll tell you what the woman told me today, a place where she said she saw people picking a few days ago:
She said to drive up highway 80 toward Richmond from Berkeley and get off on Solano (after Central Avenue, not the one in Albany). When you get to the stop sign at the end of the exit, turn left and go past Solano and to the next stop sign, where you turn right. You should see a sign or signs telling you you are going to Wild Cat Canyon and proceed there, where you will find the bushes.
I was told by a guy I met a few weeks ago that there are loads of blackberries near the San Geronimo Golf Course in San Rafael. I remember seeing massive growths of blackberry bushes along a highway I was biking some years ago in Marin County, north of San Rafael. I
was going to trek to Marin about now, but decided to try Berkeley first.
Some hints:
1. Take a picking container, have it hung in a harness around your neck so it will be hands free. I tied a shoe lace around a quart yogurt container and pinned it to a piece of nylon webbing with safety pins.
2. Have a glove on your non-picking hand, light leather is ideal. Blackberry bushes are covered with sharp thorns which besides puncturing you can leave little slivers in your hands. Today, I had the idea that I should have brought a tweezers with me. I needed one a few times, but got by using my finger nails. However, you should not have a glove on your picking hand. There are two ways to differentiate ripe berries from immature berries. The first and foremost is how they look, the second is how they feel. A glove would not permit your picking hand to detect whether a berry is ripe or not, and it would not permit you to have the necessary tactile sensations to pick well.
3. Wear a long sleeve shirt.
4. Don't pick where a lot of cars pass close to the bushes. You don't need contamination from exhaust. I did rinse my berries this afternoon before making jam.
I had a large backpack on, in which I kept containers I could dump my picking container into repeatedly, plus water and food if needed. I also had a pruning shears with me. I used it only a couple of times, didn't want to make a mess of things or leave a legacy of my picking foray.
Picking wild blackberries is IMO very zen. You have to be patient and thoughtful or you will keep getting punctured by thorns. You also have to contort your body in new ways to get at berries without getting poked, so it's like a free form yoga session. If you aren't in very good shape, you'll find out pretty soon!
Good luck.