This summer, I have been picking Himalayan blackberries in a local schoolyard. Twice in the last month, I developed sharp pains in my hand immediately afterward. There must be some type of neurotoxin in the bushes because the pain cannot be attributed to any cuts or scratches and is much more intense than a standard rash. What part of the plant could cause that reaction?
I checked Botanical Dermatology (Mitchell & Rook, 1979), Plants That Poison (Schmutz, 1979), the Plants for A Future database, and Toxic Plants of North America (Burrows/Tyrl, 2001) and found no references to any toxicity. Medical Botany: Plants Affecting Human Health (Lewis, 2003) describes the use of a blackberry-leaf tea for settling the stomach, so the leaves, when steeped, are not toxic. You do not say whether the pain is superficial, such as a skin rash, or deeper, which would make me wonder about some kind of stress or overuse syndrome.
Since we cannot give medical advice, you should discuss the incident with your health practitioner to see if you have some sensitivity to Himalayan blackberry, or to something else you encountered in that area. I found this link (to a page called Native Plants of Montara Mountain) showing the similarity in appearance between Himalayan blackberry and poison oak. It might be that you encountered poison oak without knowing it.
It may be that some other environmental factor (perhaps an application of herbicide) caused your distress. You should check with the school’s grounds supervisor to be certain.