To drink: a delicious trick is to serve cold apple juice or cider with a splash of Mûre (the blackberry equivalent of Cassis. Basically, this is an apple blackberry kir).
Blackberries go well in a sharp-sweet sauce for game.
Scatter them on porridge and puddings.
Macerate in Mûre or Cassis and icing sugar, chill, and serve with cream or icecream.
Blackberry and apple is a classic pairing you can put to good use in jam, pies or crumbles.
Stew cooking apples with blackberries and sugar, or stuff the apples with the berries and bake; serve with thick cream.
Chop a few chillis into your blackberries before making jam, and you have chutney.