Put the sugar and measurement cold water into a frying pan and heat gently, stirring very occasionally, until the sugar has completely dissolved. Bring to the boil, then cook for 3–4 minutes, without stirring, until the syrup is just changing colour and is golden around the edges.
Add the measurement boiling water, standing well back as the syrup will spit, then tilt the pan to mix. Add the blueberries and cook for 1 minute. Remove the pan from the heat and leave to cool slightly.
Mix the fromage frais and custard together, spoon into 6 small dishes, then spoon the blueberry mixture over the top. Serve immediately with baby meringues, if liked.