Juice the blood oranges to get 2 ounces of fresh blood orange juice (4 tablespoons or 1/4 cup).
Juice the lime to get 3/4 ounce of fresh lime juice (1 1/2 tablespoons).
In a cocktail shaker, muddle the jalapeño pepper slices, then add the blood orange juice, lime juice, tequila, and simple syrup. Top with ice.
Cover the shaker, then shake well.
Rim a rocks glass with salt (optional), then add ice to the glass. Pour the cocktail into the rocks glass and serve.