Sterilize jars, set lids in hot water to activate rubber compound.
Prepare fruit by crushing, then measure two cups of combined fruit using all of the dragonfruit and enough blackberries to fill measuring cup. Add to pan with lemon juice.
Stir in sugar. Add 1/2 teaspoon butter or margarine to reduce foaming if you’d like
Bring to full roiling boil (shouldn’t be able to stir it down from boil) on high heat, stir constantly to prevent sticking.
Stir in pectin quickly and return to full roiling boil for exactly one minute, continuing to stir constantly. Remove from heat and skim any foam before ladling into jars. Alternatively you can stir as the jam cools slightly, which will incorporate back some of the foam and help keep fruit from floating as much.
Ladle into prepared jars, leaving 1/8th inch of headspace. Wipe rims and threads before placing lids and bands, and tightening with fingers. Process in hot water canner for ten minutes. Refrigerate any unsealed jars for up to three weeks and store sealed jars for up to a year in a cool, dry, dark place.