Puree and strain watermelon and blueberries. You need a total of four cups of juice.
Chill and pour one cup of juice in a large bowl, boil the remaining three cups and honey in a sauce pan.
Sprinkle the four packets of unflavored gelatin over the juice in the bowl, let stand 1 minute.
Add three cups juice heated to boiling and stir until the gelatin in completely dissolved (about five minutes.)
Pour into a 9" x 13" (quarter sheet) cake pan and allow to chill for at least 3 hours.
Cut into sizes appropriate to your dog and serve.