Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the eggs and egg yolks, sugar and lemon rind into a bowl and whisk together until just mixed.
Pour the cream into a small saucepan, bring just to the boil, then gradually whisk into the egg mixture. Strain the lemon juice and gradually whisk into the cream mixture.
Pour the mixture into 6 small coffee cups and put them in the slow cooker pot. Pour hot water into the pot so that it comes halfway up the sides of the cups. Loosely cover the tops of the cups with a piece of foil, cover and cook on low for 2–2½ hours or until the custards are just set.
Lift the cups carefully out of the slow cooker with a tea towel and leave to cool. Transfer to the refrigerator to chill for 3–4 hours or overnight.
Set the cups on their saucers and decorate the tops of the custard with blueberries.