Place the sugar, water and blood orange juice in a medium saucepan. Cook, stirring every now and then, over high heat until the sugar dissolves and the syrup comes to the boil.
Reduce the heat to medium-low and simmer for 20 minutes, or until the syrup thickens slightly. Stir in the citric acid. Strain the cordial into a sterilised bottle. The cordial will keep, refrigerated, for up to one month.
To serve, pour the syrup into glasses, top with ice water or soda water, stir well and serve immediately.