In a gallon-sized resealable plastic bag, dissolve 1/4 cup of salt in the warm water. Add the vinegar, red pepper flakes, brown sugar, ice cubes and the pork. Brine for exactly 20 minutes. The pork becomes mealy if its brined longer.
Meanwhile, prepare the Blackberry Jalapeno Glaze.
Remove the pork from the brine and pat dry. Rub the pork with the olive oil and season with 1 teaspoon salt and freshly cracked black pepper.
Preheat a grill or grill pan to medium heat.
Add the pork tenderloin and cook for 6 minutes on each side, browning evenly and the internal temperature registers 135 degrees F on an instant-read thermometer. At this point, evenly spread 1/4 cup of the glaze over the pork. Cook for an additional 4 to 5 minutes. Remove the pork from heat to a cutting board and keep it warm tented with aluminum foil.
Slice the pork, arrange the slices on a serving platter and serve with remaining warm glaze.
Blackberry Jalapeno Glaze:
Add the butter to a saucepan over medium-high heat. Once melted, add the jalapenos and garlic and saute for 3 to 4 minutes. Stir in the blackberry preserves and 1 cup wine and combine well. Simmer until reduced by half. Add the cornstarch to the remaining 1/4 cup red wine in a small bowl and mix well. Pour the cornstarch mixture into the pan and cook for 5 to 6 minutes more. Keep the glaze warm until serving.