In a small saucepan, mix together 2 cups frozen berries, water, sugar, and the zest from the lemon (about 1 tablespoon) as well as its juice (2-3 tablespoons).
Bring the mixture to a boil and cook for 5 minutes or until liquid is reduced by about 1/4. Set aside to cool, then transfer to a sealed container and place in the refrigerator.
In a small bowl, mix together mustard, chili powder, paprika, salt, and pepper. Rub over pork and place into crockpot.
Add the remaining 1 cup blueberries, club soda, tomato paste, and 1 cup of the blueberry mixture. Cook on low for 8 hours.
Once finished cooking, remove any liquid from the crockpot and add, along with the remaining blueberry mixture, to a small saucepan. Bring to a boil and allow to slightly thicken.
Meanwhile, shred pork with two forks; keep in the slow cooker to keep warm. Cut celery and carrots into sticks.
Once the liquid has thickened, add it back to the pork and stir well to coat.
Serve pork on whole wheat rolls, with celery and carrot sticks on the side.