Place the blackberries, granulated sugar, and lemon juice into a medium-sized saucepan. Stir gently to mix the ingredients together.

Heat the saucepan over medium-high heat. Stir consistently until the sugar dissolves and the fruit releases its juices. Allow the mixture to gently simmer for about 20 minutes, or until it thickens and reaches 220°F. Stir in the cinnamon and nutmeg.

Remove the pan from the heat. Carefully pour the hot jam into a clean, sterilized jar, leaving a small gap at the top. Let the jam cool at room temperature for at least 2 hours before storing in the refrigerator.