Rinse the grapes and blueberries and drain the excess water.
Attach the KitchenAid Juicer Attachment to your stand mixer with the high pulp filter, then place a tall container underneath the juice opening and another container underneath what I call the "poop shoot."
Turn the stand mixer on high speed and start juicing! Juice the grapes and blueberries until you get ~1¼ cups grape juice and ~⅔ cup of blueberry juice.
In a nonstick saucepan, stir together the grape/blueberry juice, lemon juice, blueberry flavor, stevia extract and food colorings with a rubber spatula. Sprinkle the gelatin on top, stir lightly, then let it sit for 5 minutes.
Place the pan over medium-high heat and smush the big clumps of gelatin, but you don't need to worry, they'll dissolve.
Stir constantly until the mixture begins to bubble. When you can't stir the bubbles away, then remove from the heat.
Place your gummy bear molds on a cookie sheet, and then fill the molds with the gummy bear mixture. Let cool on the counter for about an hour, then place the tray in the fridge. Refrigerate overnight. Refrigerate to store. Keeps for ~4 days.