They can be fairly polluted; the UK Food Standard Agency (in 2006) recommended that people don't pick wild produce from the sides of busy roads, or anywhere that it looked like it might be contaminated by oil or ash. However, blackberries (or other wild produce) from quiet roads should have reasonably low contamination from air-borne pollutants.
Apply your common sense and don't pick eat berries that look mouldy or infested. Ideally you should wash the fruit before eating them, too.
Apply your common sense and don't pick eat berries that look mouldy or infested. Ideally you should wash the fruit before eating them, too.