To rim your glass, run a wedge of lime around the edge of the glass, and dip into the margarita salt.
To a cocktail shaker, add 1 cup ice, tequila, blood orange juice, lime juice, and agave.
Shake well to combine and chill.
Add 1 cup fresh ice to your glass, and pour the contents of the shaker into your glass, strained.
Garnish with a fresh blood orange slice and a lime slice, optional.