Squeeze the juice from two blood oranges and one lime into a measuring cup. Add 1-2 Tablespoons superfine sugar (start with 1 Tablespoon, and taste the juice. Add more sugar if needed) and stir well until dissolved.
Wet the rim of a glass with lime or blood orange juice. Dip the rim of the glass into a thin layer of kosher salt on a plate (optional - if you choose not to rim the glass with salt, add a pinch of kosher salt to the juice mixture). Fill the glass halfway with crushed or cubed ice.
Fill a cocktail shaker with ice. Pour juice mixture and 1.5 ounces tequila to the shaker and shake vigorously. Pour into prepared glass.
Notes
Adjust the quantity of sugar as needed depending on the sweetness of your oranges as well as your preferences.To make superfine sugar: pulse granulated sugar in a food processor or spice grinder until it forms a fine powder. Granulated sugar can be used, but it takes much longer to dissolve.