Stir together the egg yolks, wine and sugar in a heatproof bowl set over a pan of simmering water (do not allow the bottom of the bowl to touch the water).
Whisk the mixture in the bowl using an electric whisk until thick and doubled in volume.
Place the blackberries in an ovenproof gratin dish and pour the zabaglione mixture over the top. Grill for 4-5 minutes, or until the top of the mixture bubbles and turns golden-brown. Serve immediately.