In a medium bowl, combine buttermilk, berries, herbs and pepper. Marinate the pork tenderloin in mixture for 12 hours.
Preheat oven to 350°F (175°C). Remove tenderloin from marinade and pat dry with towel. Season the tenderloin with salt and pepper. Heat an oven-safe skillet on medium-high heat. Sear tenderloin on all sides then place skillet in oven and cook until medium, approximately 8-10 minutes.
Remove from the oven and let rest on a rack for 5 minutes before cutting.