Gradually whisk pectin into juice in a medium saucepan until dissolved. Bring to a full rolling boil that cannot be stirred down, over medium-high heat, stirring frequently. Boil hard 1 minute, stirring constantly. Remove from heat.

Add prepared blackberries into hot pectin mixture, stirring vigorously for 1 minute. Stir in sugar, granular no calorie sweetener or honey, all at once.

Ladle the jam into clean jars leaving 1/2 inch headspace. Secure the lid and let jam stand in refrigerator until set, approximately 24 hours. Serve immediately, refrigerate up to 3 weeks, or freeze up to 1 year.