Earlier posts identified these berries as native Pacific Blackberries. However, a berry connoisseur neighbor says these are Boysenberries, not blackberries.
There are 188 species of berries listed on Calflora, including several native and non-native species that look just like this plant. So the ID is not definite yet.
Disclaimer: The Corralitas Red Car Property Blog prefers to quote reliable sources, such as Calflora and California Poison Control, on the issue of toxicity. We often get email suggesting some plants are edible. If you are eating your way through the Red Car Property, we DO NOT recommend you eat any plant you find in the Red Car Property neighborhood without first doing your own research.