Fresh and frozen blackberries can be made into juice concentrate and then frozen or canned to preserve it until you want to make jelly or defrost some for a beverage. Ice trays work well for freezing and storing small portions of concentrated blackberry juice, but if you don't want to bother with defrosting the concentrated juice, simply can it in glass storage jars that you can open and use immediately.
Making the Juice Concentrate
Place fresh or frozen blackberries in a saucepan with a thick bottom to prevent burning. Add just enough water to make the berries float in the pan.
Turn the heat to medium and cook the berries, stirring occasionally, until they reach a gentle boil. Mash the blackberries with a potato masher or similar utensil while they are still in the saucepan.
Bring the blackberries back to a gentle boil and remove the pan from the burner. Allow the berries to cool for a few minutes while you prepare to express the juice.
Cover the inside of a colander with several layer of cheesecloth. Place the colander over a bowl to catch the juice. Pour the mashed blackberries into the colander and allow the juice to drain through the cheesecloth. Leave the berries in the colander until the berries are cool enough to handle. Twist the bag closed and gently squeeze out any excess juice remaining in the berries. Use a jelly bag in place of the cheesecloth if you have one.
Stir in your choice of sweetener, such as sugar, honey or artificial sweetener, to taste. Stir well until the sweetener is completely dissolved.
Allow the expressed, concentrated blackberry juice to cool to room temperature. Pour the concentrated blackberry juice into ice trays.
Set the ice trays in the freezer against the freezer walls, where they'll freeze fastest. Allow about 6 hours for the juice to freeze solid in the trays.
Transfer the frozen blackberry juice cubes to a freezer bag, leaving about 1 inch of head space so you can close the bag. Squeeze out as much air as possible and seal the bag. You can place the bag of juice cubes inside a plastic freezer storage container for additional protection, but this isn't necessary.
Label the bags or containers with the storage date and the expiration date (roughly eight months after the storage date).
Pour the concentrated blackberry juice into clean, sterilized canning jars. Use smaller jars if you only expect to use a small amount of the juice at a time.
Place a new canning lid over the mouth of the jars and tighten the lids down with a metal band.
Transfer the jars to a hot water bath canner, a large pot of simmering water with the water level falling just below the level of the metal bands. Process the jars for 10 to 15 minutes in the hot water bath.
Remove the jars from the water bath and set them on a counter to cool. As the juice cools, the button on the lid is sucked down, creating a vacuum seal. You can remove the metal bands after the jars are completely sealed. Store the jars at room temperature until ready to use.