Place the berries in a colander/strainer and rinse them thoroughly under cold tap water. Shake off as much excess water as you can.
Fill a bowl with the cleaned frozen blackberries, but leave some room for water.
Fill the rest of the bowl with cold water (not warm or hot water.)
Set a timer to go off in 5 minutes.
Gently squeeze a blackberry to feel if it's still frozen. If not, replace the water with more cold water and set another timer to go off in 5 minutes.
When the blackberries have thawed, drain the water.
Gently pat the berries dry with a dark towel or a paper towel.