Check the berries and remove any mushed, discolored, underripe or overripe berries before you freeze them.
Line a baking sheet with waxed or parchment paper. Cover the tray with a single layer of blackberries, but don't overcrowd them:
Slide the tray(s) of blackberries into the freezer and allow them to freeze for at least 3 hours. Or, just leave them to freeze overnight.
Take the baking tray with the blackberries out of the freezer. Put the blackberries into a resealable freezer bag.
Gently press the air out of the freezer bag(s) and seal them.
Straw "vacuum seal" trick (optional): Slide a straw into the bag and suck out the air so the bag "shrinks" around the blackberries. Slide the bag down the straw as you inhale, and quickly seal the bag while you remove the straw.
Write today's date on a freezer label, stick it to the bag, and place the bag in the back of your freezer.