In a small sauce pan, add 1 cup of blackberries, rosemary, vinegar, water, salt, and pepper. Turn heat to medium-high. Once the mixture begins to boil, turn heat off and let the mixture cool. If blackberries are still whole, give them a little mash with a fork. Reserve 1/4 cup of the mixture and set aside for later.
While the mixture is room temperature but no longer boiling hot, begin to prepare the tenderloin. Place tenderloin in a gallon-sized plastic bag. Add marinade to the bag with the pork and massage into the pork until pork is covered in marinade. Place in fridge to marinate for at least 1 hour. I don't like my pork too soft and tender once it's cooked so 1 hour is my maximum (it's the texture of steak at this point). But if you like it super tender, marinate for 4-8 hours or even overnight.
When ready to cook, preheat grill on high heat. Remove tenderloin from marinade and discard marinade. Place tenderloin on preheated grill over open flame. Grill for 7 minutes on one side. Flip to other side for 6 minutes. Then turn grill off, but keep lid closed shut. Let pork sit for 5 minutes. Remove from grill onto cutting board.
Let pork sit for 5-10 minutes to rest. In the meantime, prepare the sauce. Place 1/4 cup of reserved marinade from Step 1 into small sauce pan. Heat to medium heat until it's boiling. Add garlic and continue to cook until it's reduced by half. Mix water and cornstarch together to make a slurry. Add to mixture and whisk together. Add blackberry preserves and whisk to thin it out. Keep stirring and cooking for 1-2 minutes, until it is thickened. Remove from heat and add last 1/4 cup of blackberries.
Slice pork into 8-12 slices. Place a little sauce on plate and then serve each person 2-3 slices of pork. Garnish with a little extra rosemary and blackberries, if desired. Serve with your favorite Clos du Bois wine. My husband loves the Clos du Bois Chardonnay but I like the pairing with the Clos du Bois Cabernet Sauvignon.