Meanwhile to prepare the blueberry barbeque sauce, heat the oil in a medium saucepan over low heat. Cook the onion and garlic, until softened. Add the passata, blueberries, sugar, vinegar, Worcestershire sauce, cumin and cayenne pepper and bring to the boil. Decrease the heat to low and gently simmer for 10 minutes, or until reduced and thickened slightly. Puree using a handheld blender. Set aside to cool.