Remove plum pits: Remove the pits from the leftover plums. Discard any with wrinkled or hard skins. Use only the soft skins and flesh. If you like, finely chop the plum skins.
leftover plums & spices from plum syrup
Weigh and prepare: Weigh the pitted plums, then prepare three times that amount of blueberries. Measure sugar as 50-70% of the combined weight of plums and blueberries.
blueberries, sugar
Mix sugar and fruit: Place the leftover plums, spices, and blueberries in a bowl and sprinkle sugar evenly over them. You can also put everything directly into the pot you will use to cook the jam.
Mix and rest: Stir thoroughly so sugar is well distributed. Let the mixture sit for about 30 minutes until the blueberries release their juice.
Sterilize jars: While the sugar is blending in, sterilize the storage jars. Place jars, lids, and water in a pot, bring to a boil, then simmer on low heat for 5 minutes. Remove carefully and place upside down on a rack to dry.
Simmer the jam: Once the fruit has released enough liquid, transfer the mixture to a pot and simmer gently on very low heat. Stir constantly to avoid burning or over-thickening. When the jam looks glossy and still moist (refer to the cold water test for readiness), turn off the heat.
Jar the jam: Fill sterilized jars with the hot jam, leaving about 5-10 mm space at the top. If the spice pieces bother you, remove as many as possible with a clean spoon before filling.
Vacuum seal for long storage: For long-term storage, vacuum seal the jars.