Rinse fresh blackberries or thaw frozen blackberries, mash with a potato masher in single layers until fully crushed. You can leave the seeds in your jam, but if you prefer less or no seeds, after mashing, put berries through a fine mesh sieve. Measure mashed and/or sieved berries to have 3 ½ cups.
Pulse the mango in food processor or very finely chop, measured out to have 1 ½ cups mango, add to the mashed blackberries, for a total of 5 cups of fruit.
Place berries and mango into a 6-8 quart saucepan. Add ground cardamom to blackberry mango mixture. Mix until fully combined. Add fresh squeezed lemon juice. Gradually add powdered pectin. Add sugar and mix until fully combined. Bring mixture to a full rolling boil over med-high heat, that doesn’t stop boiling when stirred. Add full measure of sugar, stirring to dissolve. Return mixture to full rolling boil and boil hard for one minute, stirring constantly.
Remove from heat, stir and skim any foam off of top.
Pour jam into hot, sterilized jars, clear jar rims and thread tops of any jam residue with a clean, damp cloth. Apply lids to jars, then apply bands and adjust until fit is fingertip tight. Process in a water bath canner for 10 minutes following home canning instructions from a recognized source such as the National Center for Home Food Preservation.