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.
Notes
An alternative: Use homemade jalapeño simple syrup instead of regular simple syrup. If you choose this route, you can opt for no fresh muddled jalapeño (or less of it). For an even spicier option, try habanero simple syrup. Another alternative for a spicier cocktail is to use serrano peppers instead of jalapeños.