Muddle the jalapeño slices in the bottom of an empty shaker. Add the tequila, blackberry jalapeño simple syrup, lime juice, and Cointreau. Fill the shaker with ice and shake vigorously for 10-15 seconds until well combined and chilled. Pour into a chilled margarita glass rimmed with Dynamite salt. Garnish with fresh blackberries and a jalapeño slice.
Using a small saucepan, combine the sugar and water. Stir well and allow the mixture to come to a boil over medium heat. Allow the mixture to boil for 2 minutes. Remove from heat and add the blackberries. Smash the blackberries with a spoon or potato masher. Allow the mixture to cool completely. Once cool, strain into a covered jar and store in the refrigerator for up to a month.