Blackberry bushes are invasive and grow fast. If you forgive the bluntness, having a source of food/fertilisation, the blackberries could have grown to cover a body very quickly, easily in the time since Jade's disappearance and presumed death.
Not a botanist, I just lived for many years in a rural village with a lot of previously developed land that had fallen fallow. Blackberry bushes were a major, major invasive weed that required constant attention from the parks department folks responsible for the area.