Add the salt and sugar to a shallow dish. Mix with your fingers to combine.
Moisten the edge of the glass with lime or blood orange. Dip the rim of the glass into the salt/sugar mixture.
Combine the blood orange juice, lime juice, agave, tequila and ice. Shake to combine.
Pour the margarita into the rimmed glass and enjoy! Garnish with a slice of blood orange, if desired.
Can't find blood oranges? You can absolutely make this recipe with regular oranges! Just omit the agave, as orange juice is naturally sweeter.