Mix the pectin crystals with 1/2 cup sugar in a bowl. Stir the blackberry juice, pectin mixture, green jalapeno, and red jalapeno together in a saucepan; bring the mixture to a boil for 1 full minute. Add the 3 1/2 cups sugar and return to a rolling boil until the sugar has fully dissolved, about 1 minute. Remove from heat; stir while off heat to remove bubbles and foam for about 5 minutes.
Ladle into sterile jars leaving 1/4 inch headspace. Seal jars in a hot water bath. Refrigerate jelly after seal is broken.