Combine the blood orange concentrate and water in a small saucepan (2 to 1 ratio). Add ½ cup sugar, a small pinch of salt, the rosemary sprigs and bring to 180 degrees on the stove (no higher). Chill completely before using in the margarita.
To make the blood orange-rosemary margarita
Combine blood orange-rosemary syrup, lime juice, tequila and Grand Marnier with ice in a cocktail shaker. Shake until chilled and strain into a salt+sugar rimmed ice-filled glass.
Garnish with a rosemary spring and an orange wheel. Cheers!
Notes
Recipe blood orange syrup recipe makes ½ cup. - The syrup can be stored covered in the refrigerator for up to a week or in the freezer for 6 months.
If it is blood orange season; you can make a syrup with freshly squeezed blood orange juice. Keep in mind that the ratio of juice to sugar will be different because it isn't a concentrate.
If the margarita is too tart for your taste; you can add a little agave syrup to the mix.