Combine blackberry juice and powdered pectin in a flat-bottomed kettle, stirring well; bring to a rolling boil, stirring constantly. Add sugar; return to a rolling boil. Boil 1 minute, stirring constantly. Remove from heat; skim off foam.
Quickly ladle jelly into hot sterilized jars, leaving 1/4-inch headspace. Cover with metal lids, and screw bands tight. Process jelly in boiling-water bath 5 minutes.
Note: If jelly is too soft, use 1/4 to 1/2 cup less juice in the next batch. If jelly is too firm, use 1/4 to 1/2 cup more juice.