In a small saucepan combine the sugar, water, orange juice, cornstarch, Grand Marnier, and orange zest, and stir well. Cook over medium-high heat, stirring occasionally, until thickened, about 5 minutes. Stir in 1 1/2 cups of the blueberries and simmer the mixture, stirring occasionally, until the berries burst and the sauce thickens, about 5 minutes. Add the butter and remaining 1/4 cup of blueberries and stir until the butter is melted and the berries are warmed through, 1 minute. Remove from heat and cover to keep warm until ready to use.