Prepare the Fruit: Rinse the blackberries under cold water and drain them. Peel, core, and finely chop the apples.
Cook the Apples: Put the chopped apples and enough water to cover them halfway in a big pan with a heavy bottom. It will take about 10 to 15 minutes of cooking on medium heat for the apples to get soft.
Add Blackberries & Lemon Juice: Pour the blackberries and sprinkle the lemon juice into the pan containing the apples. Cook on medium heat, stirring occasionally, until the blackberries start to soften and release their juices, about 10 minutes.
Add Fruit Syrup: Pour in the fruit syrup and stir well to combine. Bring the mixture to a boil by turning up the heat a little. Then, reduce the heat and simmer, stirring frequently to prevent sticking, for about 45 minutes to 1 hour, or until the jam has thickened.
Test the Jam: Put a small amount of jam on a cold plate to see if it’s set. When you push on it with your finger, it should raise.
Jar the Jam: Sterilize your jars and lids in boiling water for at least 10 minutes. Carefully pour the jam into the hot, clean jars. Leave about 1/4 inch of room at the top.
Wipe the rims clean, seal with the lids, and process in a boiling water bath for 10 minutes to ensure a good seal.
