Put the frozen cherries in the food processer and process until cherries are very finely chopped.
Place all the ingredients above (except for the grape juice), including the processed frozen cherries, in a large saucepan.
Bring to a boil over medium high heat.
Reduce to low and simmer 20 minutes or until the liquid reduces by 1/3.
Strain the cherry mixture into a small pitcher or jar.
Cool to room temperature then refrigerate.