In a large saucepan, add the plums, black currant juice, sugar, cinnamon, and lemon juice. Bring just to a boil and stir constantly for 5 minutes.
Reduce heat and simmer for 30 minutes or until the mixture reaches 220 F, stirring frequently to prevent sticking. Use a candy thermometer to check temperature.
Ladle into clean and sterile jars leaving 1/4″ headroom, then wipe rims and threads with a clean, damp cloth. Seal with lids and bands and process in a hot water bath for 5 minutes.
Remove from the bath and when cool enough to handle, label and date. Store in a cool spot out of direct light for up to one year.