Season the pork tenderloin on all sides with salt, pepper, sage, and rosemary. Place the tenderloin into a slow cooker, and spoon the blackberry jam, 1/4 cup honey, and 2 tablespoons of red wine over the pork. Set the cooker to Low, and cook until very tender, 4 to 5 hours.
About 15 minutes before serving time, pour 1/2 cup red wine, 2 tablespoons of honey, and the fresh blackberries into a saucepan. Bring to a boil over medium-low heat, and simmer until the sauce thickens slightly and some of the berries burst, about 15 minutes.
To serve, slice the tenderloin and spoon blackberry-wine sauce over slices.