Place the blackberries in an 8- to 10-quart heavy pot. Using a potato masher, crush the berries slightly. In a small bowl, combine 1/4 cup of the sugar and the pectin; gradually stir into the berries. Stir in port and cloves.
Bring to a full rolling boil, stirring constantly. Add the remaining 5 1/4 cups sugar. Return to a full rolling boil, stirring constantly. Boil, uncovered, for about 1 minute or until jam sheets off a metal spoon, stirring constantly. Remove from heat. Quickly skim off foam with a metal spoon.
Ladle hot jam into hot, sterilized half-pint canning jars, leaving a 1/4-inch headspace. Wipe jar rims; adjust lids. (If you have any extra jam, place in an aitright container and store in refrigerator for up 2 weeks.)
Process filled jars in a boiling-water canner for 5 minutes (start timing when water returns to boiling). Remove jars from canner; cool on wire racks until set.