Combine soy sauce, honey, blackberry jam, hoisin sauce, fresh ginger, garlic, chopped onion and chili flakes.
Place chicken breasts in slow cooker and pour sauce on top. Cook on low for 4-5 hours.
Remove chicken from slow cooker and shred, set aside.
In a small bowl, combine 3 tablespoons of water and cornstarch. Place liquid from slow cooker into a saucepan and bring to a boil. While stirring the liquid, add cornstarch mixture and stir until thickened. Let boil 1 minute.
Pour sauce over shredded chicken and stir to combine.