Preprep jars. Crush blackberries: half the mixture can be sieved to remove the seeds. Add berries, sugar and butter (to control foaming) and bring to a rolling boil over high heat, stirring constantly. Stir in pectin and return to boil for 1 minute before removing from heat.
Skim foam from the surface with a metal spoon and ladle immediately into prepared jars. Water-process jars for 10 minutes.