Mix pectin crystals with 1/2 cup sugar in a bowl. Stir blackberry juice, pectin mixture, green jalapeño, and red jalapeño together in a saucepan; bring mixture to a boil for 1 full minute. Add 3 ½ 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.