Place beef in slow cooker, season with salt & pepper.
Mix blackberries, balsamic, broth, garlic, onion, sugar, soy, thyme.
Pour over beef and cover.
Cook on low until shreddable (about 6–8 hours).
(Optional) Mix 8 g cornflour with 15 ml water, stir in, and cook uncovered for 20 mins to thicken.
Shred beef in the sauce and serve with creamy mash.