Choose plump, unbruised, relatively firm fruit.
Look for berries that are uniformly black without sour red seeds. 

Keep in mind that blackberries do not ripen further after they are picked. They are best picked in the morning: they will be sweeter and will keep better.

Wild blackberries are generally smaller than cultivated ones.

Since they are fragile and very perishable, it’s best to store blackberries in the refrigerator, for 2 to 3 days at the most, without packing them tightly. Sugar them lightly if you wish to keep them a little longer. 

To freeze: place the berries on a baking sheet and freeze them flat before placing in freezer bags. 

Blackberries don’t stand up well to washing, since they absorb water quickly and soften. However if you do wish to wash them, leave them in their little plastic basket to avoid handling them and pass them briefly under cold running water.

Like all berries, blackberries don’t withstand long cooking, which will reduce them to purée. If you wish to keep them whole, just warm them instead of cooking them. 

• Used in jelly, pulp, syrup, tart, clafoutis, Bavarian.
• Under the broiler: in a gratin dish, sprinkled with brown sugar and briefly passed under the broiler.
• In jam: to spread on country-style bread
• In sauce: to enhance the flavor of game or roast duck

Blackberry-Raspberry Tart
Line a tart mold or pie plate with sweet pastry and bake. Brush the pastry with red currant jelly. Whip some heavy cream to stiff peaks, adding some icing sugar for sweetness and stability. Fill the tart shell. Arrange concentric circles of blackberries and raspberries in the cream. Refrigerate.