Combine all ingredients in a medium sauce pan. Bring to a boil and reduce to a simmer for 20-25 minutes.
Transfer to a blender or food processor – you could also use a hand immersion blender – blend until smooth. If you would like the blackberry seeds disposed, send the sauce through a sieve to discard seeds.
Mix brown sugar, chili powder, cayenne pepper, garlic powder, onion powder and salt in a small bowl, set aside.
Season pork butt with a liberal dousing of salt.
Heat a large sauté pan over a high heat, add oil and brown all sides of the pork. Sprinkle onions in the bottom of your slow cooker, pour chicken stock over.
Dry pork off and rub the spices all over the pork, place on top of onion.
Turn the slow-cooker on low and cook for 7-9 hours until pork is fork tender.
Shred pork and add ¼ - ½ cup barbecue sauce, mix until combined – (This step is optional.) Serve pork on biscuits or large onion buns and lots of extra barbecue sauce.