1. Place the gelatine in a bowl of cold water and leave to soften. Meanwhile, put the cornflour in a saucepan and gradually blend in a little of the milk until smooth. Stir in the remaining milk. Add the sugar and cream, then heat and bring to the boil, stirring continuously with a wooden spoon until thick and beginning to boil up.
2. Lift the softened gelatine out of the water and lower it into the custard. Stir until dissolved. Tip in the chocolate and leave for a few minutes, stirring frequently, until melted. Pour into a jelly mould with a capacity of about 800ml. (Or use individual moulds or a small trifle dish.) Leave to cool completely, then chill for at least 4 hours or until the blancmange is set.
3. Put the blueberries in a bowl and prick all over with a fork to release the juices. (You needn’t be too precise with this, simply prod the berries roughly.) Add the sugar and a squeeze of lemon juice and leave to stand for about 30 minutes, stirring frequently until the sugar has dissolved and the juices are syrupy.
4. If using a jelly mould, fill a large heatproof bowl with very hot water. Loosen the top edges of the blancmange from the mould with the tip of a knife. Lower the mould into the water and leave for a few seconds. Lift out, rest a flat serving plate on top and invert. Shake the mould until you feel the blancmange loosening then lift away the mould. (You might need to re-dip the mould several times; metal and plastic moulds will be quicker to loosen than glass or ceramic. Take care not to dip them for too long or the blancmange will melt.)
5. Spoon the blueberries over or around the blancmange to serve.
This is best made on the morning you want to serve it — if made the day before, the gelatine will go on firming up the texture of the blancmange.