Though they are drought-tolerant plants, blackberries will produce the best fruit if they receive 1-2 inches of water a week while berries are present. Water deeply a couple of times a week when plants are first getting established and during hot, dry weather. Mulch around the plants to conserve moisture. Watering is rarely necessary in winter or while the plant isn’t fruiting.
-John
(information for example gathered from https://www.southernliving.com/garden/growing-blackberries#:~:text=Though%20they%20are%20drought%2Dtolerant,the%20plants%20to%20conserve%20moisture.)