Combine (No need to chop anything, just throw it in whole. The sauce will be strained.) garlic, blackberries, apple cider vinegar, brown sugar, beef stock, thyme, rosemary and 1 teaspoon salt and 1 teaspoon black pepper into a 3 quart saucepan and bring to a boil.
Once boiling, reduce heat to simmer and let simmer 30 minutes.
Go ahead and preheat oven to 325°.
While the sauce is cooking, on your stovetop, put 1/3 cup vegetable oil in your braising pan or Dutch oven and turn the eye onto medium high heat.
Use the remaining 1 Tablespoon of Salt and black pepper to season the outside of the pork tenderloin, on all sides.
Once the oil is rippling in the pan (just give it a few minutes and it will be ready), place your pork tenderloin(s) in the pan and let them sear on all sides. To let them properly sear, leave the meat alone until it moves easily on it's own from the pan. If you try to jiggle it in the pork and it is firmly stuck, it's not ready to turn yet. Continue flipping until all sides are browned and seared.
When the pork is done searing (not cooked through) remove from heat. Put the pork on a plate to the side for a moment.
Once your sauce has cooked for 30 minutes, pour it through a fine mesh strainer into a heat safe bowl or another pot. Discard the contents of the strainer and pour the freshly strained sauce into the pan where the pork has just finished cooking.
Stir the sauce around in the pan and scrape up any pieces of pork and brown bits that are stuck to the pan (this is flavor!).
Gently put the pork back into the pan with the sauce and pour any juices from the plate into the pan as well.
Put the lid onto the pan and put it into a 325° oven. After 1 hour 15 minutes, take the pork out of the oven and turn it over and return to oven.
After another 1 hour and 15 minutes, remove pork from oven and let rest with the lid on for 10 minutes.
Remove the pork from the pan and put on a plate and cover well in aluminum foil. The meat will rest and this lets the juices redistribute.
Meanwhile, put the pan on a stove eye over medium high heat and let the remaining sauce come up to a boil.
In a small container, mix 1 Tablespoon cornstarch with 1 Tablespoon cold water. Whisk until a smooth liquid forms with no lumps.
When the sauce boils, add in the cornstarch slurry and immediately turn the heat down to medium low. Stir with a whisk until the sauce thickens. Remove from heat once thickened.
Serve over sliced pork tenderloin and enjoy.