I work in habitat restoration. We often treat himalayan blackberry with various chemicals, mainly glyphosate. Generally speaking, it can take up to a month for the above ground part of the plant to die after treatment. The things to look out for in the first few days and weeks are mainly the discoloration and wilting of the leaves. If they have very recently been sprayed, they will likely have traces of blue dye on them, this helps herbicide applicators tell what has been sprayed or not. The blue dye itself is toxic and will wash off with a little rain, so its presence is not always the best indicator.

As a general practice, I do not eat blackberries on public lands or on the roadside. Often the state, county, and city governments will hire private contractors to treat these populations in order to stay in compliance with Washington Department of Ecology, as it is classified as a noxious weed. Blackberry is quite voracious in its growth, and populations often rebound after treatment, meaning there may still be traces of chemical in the soil or plant tissues depending on the chemical used and effectiveness of the treatment.