This is a good time to carry out control of this invasive weed. If you wait until the berries form, birds and foxes will eat the fruit and continue the spread across the countryside. Don’t put off weed control, start with isolated small plants then move onto the larger clumps. If you can’t afford to spray all the weeds at least start by spraying around the edge of larger clumps to stop the outward spread.
If you like to collect and eat blackberries make sure they haven’t been sprayed.